Sorry you're going through these struggles. I can relate to much of what you're saying. I've seen therapists on and off since high school and I'm 36. Right now, I only see my psychiatrist. Therapy only worked as much as I was willing to work it. They would give you the tools, but you would have to use them. Two of the more practical strategies that stuck in my mind out of my hours in therapy are these:

For every negative thought you have, consciously replace it with two positive thoughts. (Something you're grateful for, noticing something beautiful, focusing on another's positive attribute, or a strength you have) One positive thought to cancel out the negative one, and a second positive thought to gain the upper hand. Just force it if you must, it may be very difficult, like a war inside your head, but over time it gets easier.

The second strategy somewhat mirrors some of the above comments. Writing your redundant thoughts down. One therapist once told me that when we dwell on a thought, it's like a spinning quarter. Sometimes we have one quarter spinning, sometimes multiple quarters spinning. When you write each thought down, it's like slapping each quarter down, and it truly can give you more peace.

Stay strong!! I know you can overcome this @ameliabartlett