First of all, try not to beat yourself up about screwing up or making the wrong decisions. This is all normal and a part of your experience. What you have gone through may need some time to process.

Start with small steps. Over thinking and over reasoning is a way for your brain to avoid what you are actually feeling. This is normal because we are all inclined to avoid unpleasant feelings. Slowly try to get in touch with those feelings, even if they're painful. The more you try to avoid them, the more they will linger, and the more you try to suppress them, the deeper your depression. Getting in touch with those painful feelings are actually not as bad as you fear. It can actually be cathartic.

Along with all the suggestions to see a qualified therapist, I would recommend reading some of Pema Chodron's books. There's one in particular that may be relevant for your situation called 'When things fall apart'

I won't wish you good luck, because luck will not make things better for you. This will take some time and a little effort to work through. You have already taken the first step by reaching out. What you do next, and the skills and lessons you will learn through processing this experience will grow you as a person. Remember - one step at a time.