I've shared similar feelings that you describe here - particularly about the wanting to be independent (not promiscuous) and move to another city so as not to always wonder "what if?". I think these things are really valuable in life and I weight them heavily. I think that a lot of people seem miserable in life and I think it's exactly because they never took any risks (no risk, no reward, right?).

My concerns about the future with my current boyfriend have nothing to do with ambition (we're both extremely ambitious, he more than me), but other things (he's controlling at times, not very social).

Luckily for me, I have more time to decide how much these things are worth to me because I will graduate at the end of this year and where I look for careers will be my "deadline" approaching. However, I think it's easier for you to ignore your deadline than it is for me. That is, I think you should not let the lease situation pressure you to make a decision. If you end up signing a new lease and decide mid-way through that you do want to be on your own, then so be it, you know? You'll figure out the details as they come. So, my advice is to take the pressure off with any "deadlines".

Finally, I'm older than you by about 5 years (and female, which matters in this analysis) and I'm particular about who I like, so it's a little scary to go be independent at this age if I choose that route. However, I'm not a big believer in "the one", and I think eventually we will find someone who we enjoy who meets certain criteria for a life-long partnership. Moreover, we'll love ourselves better because we accomplished certain goals/dreams we had (or at least tried). I think knowing and loving yourself first is also really important before trying to love someone else. I think, in the end, it helps lead to the right/better-matched person as well (maybe someone else who's done the same thing -- moved to another city just to do it).

Good luck.