Back to the OP,
[MENTION=14498]Cloud of Thunder[/MENTION], what could be happening if you're an INFJ is that your tertiary Ti is coming out at this stage of your life, and this is making you wonder if you're a Thinker. One of the popular theories of how the different cognitive functions become developed in people's lives is that at about the age you are now, that's when the tertiary is developed. This is just a possibility.
With the difference between Te and Ti, some broad differences between them is that Te tends to look outward more for figuring things out--Te-users have a tendency to quote or link to studies, scholarly articles, and things like that. They also tend to tell others their theories in hopes that others will tell them what they think--either agree or disagree so that the theories can be tested or refined for accuracy. Te is also more concerned with things that are useful in getting a goal accomplished, and generalizes more than Ti.
Ti is more about internal logical rules than looking externally. Its goal is not so much getting a goal accomplished but is more concerned with 100% accuracy. It doesn't like to generalize as much, and Ti-users use phrases like "I think...", "I believe...," "Maybe...," more often than Te-users in general.
Another difference that you can tell when you look at people's posts on this site is that Te-users tend to post in a way that's giving bullet points. The point they're making will be easier to spot. Ti-users instead tend to give paragraphs instead of bullet points, and the point they're making is less highlighted and is more buried inside of the paragraph as a whole.
Also, note that because I'm a tertiary Ti-user, I felt compelled to add that these were "broad differences," for accuracy--a Te-user would be less likely to say this but feel that it is understood that it is a generalization.
For Fe/Fi, this has been endlessly debated on this site for a long time about what the definitions of these are and how they work in people. I would recommend checking out some of the long threads that explore this topic and seeing which one you feel resonates with you the most. My favorite Fe/Fi thread is
this one.