Hello, ill make this question brief

I used to be depressed. Flat emotions. Really into routine.

This last year ive changed a lot, read a lot, been happier overall, but...

For some odd reason, i have this "streaks" where I feel happy, calm, in control of myself, decided, clear thoughts, empowering thoughts, really social, with a general feeling of well being, and alive, but this streaks they never last.

Then some intrusive thought comes, or a experience, and then its like my whole state of mind switches. It doesnt matter if i rationalize, it doesnt work if I keep thinking the positive affirmations, i just stop "feeling them". I make many efforts to go back to the positive but i cant. My response to situations changes. If, for example, someone laughs far away, when my state of mind is positive my brain sends me a "its cause im awesome" thought, but if im in the negative, a thought like "he must be laughing at me" pops, and i have to conciously think "no, its stupid to think like that".

Why?? When im in the neg i get lots of anxiety. Its like a loose control of myself. Then maybe some good experience comes and I switch to the positive and everything is rainbows. I start to think " finally ive stayed like this". And then boom, some day I go back. Its like, in one state im completely present and positive, and in the other im completely in my mind, and i cant get out.

I wish I could stay in the positive, why cant I.its like my brain switches "selfs" :S. Whats happening.