Am I doomed to keep sabotaging all of my relationships?

I’ve noticed this trend over the past five or so years..

I find really nice guys.. and they really like me… and we start dating… and then I find reasons to end it. I’ve been in three serious relationship in this time span (a 2 year, almost 2 year, and slightly over 1 year relationships) and they all seem to end the same way. Over time, I just lose interest. Nothing that they do is good enough anymore, and I just distance myself from the relationship. And then one day I just decide to end it..

I don’t know what my problem is. I really am incredibly lucky with every part of my life, yet it’s like nothing is ever good enough. And the men in my life seem to be the ones that take the hit for this.
I know it’s me, which is really not what they want to hear when I pull the whole, “It’s not you, it’s me…” thing, but that really is true. Has anyone else experienced this? Is this one of those unfortunate parts of growing up and finding the right person? Or should I spare the next few guys and just stop dating for a while to deal with my own issues?
I just really want to hear stories and outside perspective.. as I think my friends are sick of hearing about how I’m unhappy with my seemingly idyllic life.