I don't think what's happening is that abnormal -- you are in a kind of unhappy situation with your current boyfriend which makes your nice ex seem more appealing.

It can be really, really hard to dump someone who is 'too nice.' Sometimes people look really good on paper or as friends, but there's just not enough chemistry or whatever to keep a romantic relationship alive.

Smothering, jealousy, and the inability to let go of the relationship after all these years --- you had good reasons for ending that relationship. You may have good reasons to end your current relationship as well, but unless you are madly in love with this guy and have often thought about how sorry you were to break things off with him, I woudn't go back to him.