Extremely variable feelings towards boyfriend, want stability...

I have never been all that great with relationships and am generally not that experienced with them. The two I've had in the past have crashed and burned. With them, I would tend to very easily go from being generally happy and liking them a lot to being VERY difficult and moody and not liking them at all if they committed some infraction, real or exaggerated by my perception. After the relationships were over I chalked this up to my not really loving them and not really being a great match, that I got into those relationships for the sake of being in a relationship.

This time around it's a bit different; I wasn't necessarily looking for a relationship, it happened more organically, and I think we're a better match than my last attempts. So I was hoping I wouldn't have to suffer this love/hate thing. But it looms over me all the same. If things have been nice I'm happy, and if things have been REALLY good I almost feel like maybe I might be "in love," whatever the fuck that really means, and I feel dedicated to the relationship. But if he does something that bothers me (or sometimes if I just slip into a dark mood) I feel extremely ambivalent towards him, sometimes slipping past ambivalent into dislike/inner-criticism of him, and I get very annoyed with monogamy and think about sex partners I miss, etc.

It blows over eventually, sometimes rapidly, sometimes taking a while. But I'm tired of these violent swings and I just want to feel stable; I know that in all relationships it is not as if you adore them completely every second, but I think it is abnormal to a) switch back and forth so easily and b) feel so strongly bitter and unhappy during bad times. I dunno what to do. I'm 21, and female if that wasn't obvious, he's 23, we've been seeing each other since early April.

TL;DR I go from feeling dedicated and happy and maybe "in love" to feeling angry and unhappy with monogamy and seeing lots of flaws in my boyfriend, an effect which I've had in every relationship so far. Help.

Edit: You guys are amazing. That is all.