I've been in 6 serious relationships spanning the past 7 years combined (from age 18 upwards). It's been enormously interesting to me and I've learned a hell of a lot about myself.
The biggest recurring obstacle for me is that I often feel guilty and incompetent in relationships, because I will invariably lean on my girl for companionship and to get things done in the real world. In that sense, I struggle with wearing the proverbial pants, even at times when I really feel I should be stepping up and taking the lead in certain areas. So even though I do okay financially and everything, I still feel like dead wood, simply because I oftentimes can't cope with the world and am not sociable. This frequently leads to big arguments, whether it's my refusal to attend a family event of hers, or to get something pragmatic done by myself. There have been phases where I've had to have my girlfriend at the time make all my phone calls for me simply because I'd get too nervous, screw up my sentences, and hang up on whoever it was. Of course, my frequent mood swings, irritability and generally imbalanced emotional state have never helped matters.
To say that SA or depression has been the killer in my relationships is exaggerated though. Mostly it's just a plain incompatibility issue. I don't struggle with jealousy or trust. It's just a men are from mars, women from venus type thing, usually. So I can't say I think about relationships a lot; I've been too busy with them since adulthood that I've hardly had any downtime (ie. being single). But again, that's more of a weakness on my part - having a dependency on the opposite sex for whatever needs I have. It's not my partner's job to be my caretaker, and I fully realize that, but breaking that cycle is easier said than done.