i think it's better to feel some guilt than to not feel guilt at all. but it's hard to not go overboard and get paralyzed by wishful thinking. i have had this problem a lot a lot.
cathylynn had a good comment, especially "the purpose of guilt is to make us change for the better. if we have done that, it's time to remind ourselves that we need to let go of the guilt until we actually do let go."
holding onto the guilt and the past can really throw you into a rut. although it would be nice if we could go back and fix things in the past, try to think of these things as life lessons, even if it's only what NOT to do in the future. (cliche right? )don't be so caught up in your past mistakes that you freeze up and make them again. sorry, does any of this make sense?
also sometimes framing it this way can help: if you had a good friend and they made those mistakes, would you be so hard on them? (this does not work for everything but usually we are harder on ourselves than others)
sorry, trying to make you feel better but i'm not sure how to word these thoughts very well or if any of this is even relevant enough! hope you feel better though