In more than one case, I have been undeniably drawn towards someone who has many attractive qualities, but on a fundamental level shares none of the elements needed to make a long term relationship work (different religion, values, priorities, attitudes about drugs and alcohol, ages and so on). I believed at the time that having a friends talk at the beginning would then allow us to still be in one another's company. Even though I was careful in the beginning to not give mixed signals and get physically involved, spending unlimited time in one another's company, with him pursuing, made the attraction grow on my part, despite serious misgivings about the sustainability (or even desireability) of the relationship. In the second case, the proximity that we shared, our living in an isolated community without much other support system, his pursuing me, and a myriad of other factors resulted in me not wanting to not see him at all, and finally realizing at some point that we either had to start dating or else not talk to each other. I took the easier of the two options, we were together five years, but with three (I know!) of those years spent having already determined that we would only be together while we were there and not after. Had you asked me if that was a good course of action, I would have readily acknowledged that it wasn't, but by then we loved each other and I was very invested, despite seeing that it clearly could not work in the long run. Had it not been for witnessing some concerning behaviours towards me, family and friends which were rooted in insecurity, recognizing those behaviours and what they meant (from watching my siblings' and some older cousins' marriages) and determining it was doomed for failure, I would likely have married him, despite all of the foundational issues that we did not see eye to eye on.
Insecurities and baggage from the past that have not been dealt with are often at the root of addictions. Until those issues have been sorted out, it is, as Kyuuei said, not in him to be a partner. I wish I had understood this sooner (alcohol was definitely a factor in the breakdown of our relationship -he used alcohol to cope more than I realized at the beginning - and yet by then I had invested enough and cared deeply enough about him, that I didn't want to break things off before I had to). My sister married a man who had deep emotional issues and who later turned to alcohol and drugs to cope. Soon after they broke up (last summer after a 20 year marriage in which she traded everything important to her for him), he died in an apartment fire started by accident by himself while under the influence of drugs and alcohol, which endangered many other people's lives. She was just thankful that she and the children were not casualties as well, as they faced many life threatening situations during her time with him. Only by her vigilance they did not end up dying under similar circumstances!
Insecurity is inherently selfish (it is a disbelief that anyone else will care for the person if they don't put themselves first) and often manifests itself by curtailing any relationships or interests that you as a partner care about (thereby isolating you). It is not the persons fault that they have experienced circumstances at a formative time that have inflicted deep wounds. It is however, their responsibility (and not yours) to heal up from them and resolve those issues. Until that time, they will not be able to absorb any of the love you are showering on them.
I feel so fortunate that my sister still has some years left of her own life and that her relationship did not claim her health, as it has many people I know. However, no one can get back the years of emotional growth that should have occurred from 20-40 (she was busy just with the business of surviving and earning them in a living while he insisted on living in an expensive city and did not work throughout the years). Neither can she get back those years of life as an extended family and the history and growth in our relationships while my parents are still living. She missed a lot of occasions (to prove her loyalty to him) that were pretty big ones in our family's life.
Of course I realize that every situation is unique. However, I would hate to see you lose years of your life to a relationship that leaves you bereft or bankrupt in areas of life that matter a lot to you.
I still find myself some days missing the good things about the five year guy and I think it's probably a very good thing that we are no longer in touch and live on different sides of the country. I understand that tug of war, but if there's anything I can offer in retrospect, it is that I would have done what seemed hard and heart-wrenching at the time, rather than waiting until it was even more difficult to do so.