Regret

Does anyone else have one of those things in their past that they just can't get over? Please tell me its not just me. I can't get past it, can't forgive myself for it, can't stop wondering what life would be like if that one thing had not occurred.
I know, somebody will say, "Mandee, the Lord has forgiven you so you must forgive yourself" or, "we are all sinners washed clean by Christ's blood."
Believe me, I know these things in my heart. I know that God loves me and that Christ died for my sins and I am clean because of that. But I'm also human and I don't know how to my head to understand that.
It seems that things come along to throw my mistakes back in my face...its like ANOTHER consequence to my action. There are constant reminders that I really screwed up. I think about it a lot and I'm not really sure how to truly put it behind me.
I just can't help but think that one action that I took years ago has changed everything about me and about my life. Some things will never be the same...never be what they could have been and I will have to go on knowing that and not being able to change it.
Regret is so painful, so hopeless, so defeating.
God has lifted me out of that, why am I still reaching back to it?