This is just from my own experience, so take it as anecdotal.
Hello everybody,
I absolutely need your help!
My ISTJ broke up with me after 4 years together, because he thinks that our evident differences (I'm a INFP) create excessive conflict inside the relationship.

Furthermore, I find that our differences complement each other and I enjoy them, but he seems so stressed by the fact that we like different things or that we have a different approach to life (mine WAY more laid back

)
There are going to be major differences for the rest of your lives, which isn’t a bad thing, especially with the TJ/FP differences. The ability to view them as complimentary rather than antagonistic is going to depend on the maturity of the individuals involved. Mature STJs (any type that is mature, actually) will learn to appreciate the differences that drew them to you in the first place. Immature ones will view you as a lifelong pity project that they are responsible for and pour all their energy into fixing what they see as broken rather than nurturance. My ESTJ openly states ESFP is the ideal type, and continually tries to push me in that direction and probably always will. He has little tolerance and appreciation for my N-ness, which he labels weird and inferior. I adore him, feel completely satisfied with my life and wouldn’t change a thing, but in general I ascribe to the idea that iNtuitives should be with iNtuitives, and Sensors should be with Sensors. I think in our case it would have made him infinitely happier, and that is my one regret.
It sounds like ultimately he appreciates your personality differences or he wouldn’t call you the love of his life. Are you sure it’s the personality differences, or your past and current behavior that is the problem?
I will try to be more clear. I and my love come from very different backgrounds (country, family, social environment, experience of life).
This is my case as well. My husband comes from an affluent, east coast background. I come from a rural, blue collar background. I worked hard for everything I have in life. Although he is hard-working, he had most things handed to him. He is very aware of the classes, and is disdainful of people from different backgrounds and perspectives. Difference in background can factor heavily for this type. I know it has factored heavily over the years for mine.
I have now a very good job and live and work in a "conservative environment", and I feel good and serene in it, but in the past I had difficulties finding my way and I did some "borderline experiences".
This is very nebulous. It seems to factor heavily into his decision-making, so it is hard to understand where he might be coming from without knowing more.
He is very strict about how life has to be lived. Tend to per-judge a lot.
This is just how STJs are. They tend to expect that they get to make the rules. This is just something you’re going to have to learn to adapt to and negotiate with if you continue together. Often it is good, because it results in growth on our part, for which I’ve always been thankful. However, it can become extremely demoralizing when you can’t perform to their expectations as fast as they demand, or don’t agree with the expectations in the first place. I’ve never been a sensitive person and take criticism well, so that helped immensely, but assertiveness with those I care for was never my strong suit. I’ve developed quite a backbone over 10 years.
He says that "the past explains the future" and he thinks that I will be back to behaviors that he disapproves.
I left my past behind me years ago, I feel happy, satisfied, and safe in my current life. But my partner try to find signs of fault in every little thing or word. I have to admit that once I lied about a mail that I sent to a former boyfriend for his birthday, as this guy was living in a very difficult situation. I didn't tell my partner because I was scared of his reactions knowing how much he is insecure and scared of my past, he discovered this mail (absolutely innocent and brief) and since then on he has created a system. Every word I say is a proof that I'm not good for him. He sees things that don't exist (that I flirt or smile to men).
You screwed yourself here, sorry.
1. These types (and most people) base trust on established behavior. Present behavior if given enough elapsed time can definitely override past behavior and demonstrate genuine change. But if your past involved lying, betrayal, and omission, no matter what the reason, and he caught you doing it again, he has every right to be distrustful. Even if he didn’t like that you were sending it, if you had told him about it, he would at least have the assurance you don’t hide things from him and you can have an honest dialogue and the integrity of the intimacy is preserved. Now you have two problems instead of one – he doesn’t like that you did it, and he thinks you’re sneaky and not safe. And once you have that problem with an STJ, good luck, you’re in for some work. They have the memory of an elephant.
My ESTJ was screwed over by an ESFP before we got together. He was incapable of trust, and says he picked me because he knew me and knew he would never be hurt or betrayed by me, and that I would never leave/abandon/reject him, and yet I still spent 8 years cleaning up after a mess that I didn’t even have anything to do with. I considered him completely worth the work needed. I am extremely transparent with him. Even if he doesn’t like something I do, he knows I don’t hide things. That is the foundation of intimacy and trust. If you yourself were/are the one demonstrating dubious behavior, you’d better mean it when you say you love him, because it’s going to take an incredible amount of work and time to fix it.
2. If it were truly innocent, you should have been able to tell him about it. And he knows this. He shouldn't hold your past against you if your behavior is clearly different now. We all make mistakes, we all can change, and we all need grace. We also need sympathy, empathy, and for people to see and appreciate and call forth the real us despite our struggles. If you have a struggle, be honest with him. Trust can be forged when transparency is there.... having a problem isn't the problem....hiding the problem is the problem.
Please don't misunderstand me. He is a wonderful person. Caring, loving, generous, but scared of everything, with the mania to control everything.
Yes, they are wonderful people. And yes, control is everything with them.
I love him sincerely, I have always respected him deeply. I want to share with him my whole life, but I fear that he is gone for good.
Is there any ISTJ that can answer my question: An ISTJ can overcome his fears of what is different for love?
I have learned that I have not to lie anymore, not even on small things, but he doesn't give me another possibility, he has gone to far, has built a system in his mind.
It took 8 years of consistent behavior on my part as well as professional therapy on his part for my ESTJ to get over something that didn’t even originate with me. You have do some serious introspection to ask yourself how committed you are to him and cleaning up any messes you’ve made.
I have learned that I have not to lie anymore, not even on small things, but he doesn't give me another possibility, he has gone to far, has built a system in his mind.
If someone will lie in small things they will lie in big things. Especially if they used to lie about big things in the past and that path is already familiar. Not lying or omitting any longer is a great start.

You just have to figure out whether you want to put the time into cleaning it up given the road might be extremely difficult with this type, whether he himself is up for the work involved, and the rigorous transparency necessary to providing the contrast between then and now.