Omg, I wrote a long message in response to this but apparently was logged out in the process and it didn't send when I tried to submit (asking me to log in again and losing my text). Ahh

Shorter version:
I'm similar to you in that I also often fall out of touch with friends and "live in my head" quite comfortably. My suggestion--make sure to meet people at least halfway most of the times. Be more proactive in relationships by initiating contact and hang outs, checking in/showing an interest in their lives & well-being while also sharing yours with them. I would think that most important is finding people with similar interests or values but I guess that's obvious & easier said than done.

I hate to bring up personality types, but you really strike me as an INTJ. Reading about your personality type (if you believe all that jazz) might help give you some peace of mind about yourself and make self-improvements. As for the ex-gf, I can relate with an old friend that I had a falling out with and eventually fell out of touch with completely. I think about her occasionally and get a little obsessive too. In any case, it seems interests didn't align with you & your ex-gf, and perhaps rightly so? My thinking is that if it wasn't "meant to be" (by that I just mean a very good match), then it's better now that it's not. It gives you each the freedom to learn from the past and find more compatible or functional matches.