How can I kill the killer in me? /chaotic mind and masochistic tendencies/?

Generally I m a very positive,open minded person.I love helping people,its makes me feel good. But lately I feel that many people just use me. I have become most of the time a serious,realistic-pesimistic.I recently found out that I have changed, I dont trust anybody,anybody at all,I doubt about everything whoever says or does.I also think about the past a lot,about the bad parts of it.I am 19 now and thorough my life I've always believed that you have to get through something to understand it and learn from it,thats why I think sometimes I have put myself in ugly scenes on purpose-to get a good lesson,I have created some bad memories that will always stay in my mind.Now I regret it,I sure learned a lot from my experience ,but the awful happenings go around in my head so often and they cause me some very bad mood ,sometimes I get edgy thoughts and I do ugly things,at times I get very depressive(cry all day long).I've become more sensitive than ever,the tiniest thing can make me cry or make my heart beat.Also-the constant blame.I blame myself for everything and take it very personal,then I hurt myself for punishment and after that I try my best to do my best so that everything with everyone is OK. But I recently found out that you cant please everyone and that makes me sad.It also makes me sad that I forgot about myself after trying to fulfill everyone else's desires.I have lost my peace of mind.How do I get it back ??How do I stop all the self-blaming?I just dont want to hurt myself again,because every time it becomes worse.I need to find another way to release this negative energy or a way to eliminate it,before it even got created. Please..