Sorry, I'm not going to directly answer your questions either, but here's the way I see your situation:
When you spend too much time with your family, do you tend to get into little petty arguments? How about with your good friends? Your answer's probably yes, and even if you don't *argue*, there have probably been times when your family was driving your nuts or a good friend was driving you nuts.
After 3 years, you've gotten to the point where you don't do the lovey-dovey stuff, like you said (and, personally, I think the couples that do WAY too much of that after a few years are covering up for a relaitonship isn't there). From my experience, early on you're just so excited to be together that you tell each other every little mundane detail of your lives and plan for your future together and basically think you're both the best couple ever.
Once you get past that and don't need to talk about how much you love each other and prove it constantly, things can get kind of boring. When people get bored, they tend to get crabby sometimes and that's when stupid nit-picky fights tend to happen. Also, if you're seeing the lack of excitement as a point of insecurity about the relationship, talking about anything around that issue might also cause little fights, especially if it's a sore subject and he doesn't see a problem or views it differently.
My suggestion would be to get something to talk about. This will probably work best if you both have different things to discuss. Take a class you've always wanted to take, hang out with your friends some more, encourage him to go out with his friends, start a hobby, anything. Sounds like most of the problem is that you're together so much and you're such a large part of each others' lives that you lost some of the individuality that you both saw in each other and liked from the start. If you start to get that back, you'll have more to talk about and will (hopefully) renew some of your interest in each other.
As for not thinking you're good enough....you've been together for 3 years. He obviously sees something in you that you might not see in yourself, so don't worry about it. If there's something you'd like to change about yourself, do it because it'll make you feel better, not because you think you have to in order to be "good enough" for someone else.