Long distance relationships are possible, but I think they only work if the relationship is well established and the individual that's not in med school understands that depending on the week, there could be a lot or a little time devoted towards the relationship. When my boyfriend has exams, I know to back off and just wish him luck and not to cause any drama or bring up things that require more than 10 minutes to talk about.
With you saying that Mr. X is a bit needy, then I would say you need to get yourself established in school, then decide if you could handle a relationship on top of your studies. Not the other way around. It's better to have looked back on your senior year and enjoyed the relationship you did have with Mr. X and just blame things on bad timing than to completely destroy the relationship and end up resenting each other because you tried and either person was too needy in the relationship. Of course, this is just my 2 cents!