Martina,
First, know that you are not alone! For every one of us (including myself) who admits to self-sabotage exactly like you describe, there are countless more who struggle in silence right along with us.
Second, there are a couple of things about your post that remind me of things that sometimes help me when I'm engaging in those types of self-sabotaging behaviors. One is to remember that the self-sabotage won't last forever. Bear with me on this one. If you read the language of your post, you'll see that you're talking about times that come and go. So if you know that you will definitely pull out of the self-sabotage ditch that we all tend to run into from time to time -- and you WILL -- maybe you can feel a little less hopeless about feeling like you're in that ditch a bit now.
The other thing I'm reminded of is that we are not equal to our mistakes. I can feel the pain as I read your post because I recognize the depth of devastation that causes a person to take such ownership of her struggles as to become one with them. Words like "Whenever . . . I F it all up again" and "I won't allow myself to be happy" and "I've given up" and "I'm incapable" and "I can't" are such heavy, burdening words, and you don't have to carry them! You are not your mistakes; each day that you struggle to learn, each day that you ask what you could do differently, is the birth of a new you, a you who is worthy of being given every opportunity for peace and happiness.
So if I were looking at myself in the mirror in this situation, I would say these things: 1) This, too, shall pass; 2) You are not your mistakes (you're so much more); and 3) Take a rest while you give the DBT a chance to work.
Big, warm hugs!
Rhonda