First off, hi I'm new! I've been lurking around these forums for a while, and they seem like fun.
Secondly, I think characters are just a matter of practice. To me it sounds like you're doing all the right things. I find it a lot easier if I write in first person perspective - you pretty much just get in the head of one person. (I'm kind of lazy, so for me this is great )
If your characters all have some kind of reasoning behind their actions and their personality quirks, then they should be believable.
I'd say you're probably being too hard on yourself. It's always easy to be uber critical of your own stuff.