Thank you so much for posting on here, it can be really hard sharing this kind of stuff, so well done for taking that step. I really related to your experiences and feelings. I too was bullied throughout school as my family was poor and dysfunctional - I got called "the pov kid" etc. Feeling like you have a bully inside your head was also how I felt for most of my teens and twenties. I now feel I have it much more under control.
For me, I needed to develop a whole set of very diverse strategies. Talking to your GP and seeing a psychologist is a really good step to take. You can also do a lot of exploration yourself of the different psychological therapies out there. I tried out pretty much all the big ones, CBT, ACT etc.
The one that was most helpful for me is called "Schema Therapy" - it's like a hardcore version of cognitive behavioural therapy (CBT). It's commonly used by clinical psychologists for tough cases where other therapies aren't effective. Like CBT, with Schema Therapy you learn to recognise dysfunctional, extreme or unjustified negative patterns of thought, and think about other ways of looking at things, but the difference from CBT is you also think a lot about the underlying deeply held traumas or experiences that may be responsible for continuously generating the negative thoughts, no matter how many times you "refute" these thoughts.
Things like Schema Therapy have worked for me over the long term. However, I also need more short term ways of quickly short-circuiting the internal bully. Even though I'm not religious, I found saying aloud simple mantras from different religions effective ways to distract my mind when I feel myself spiraling downwards. Something about speaking out loud slows my brain down.
I've also found regular intense exercise essential. Anything that forces your attention fully onto your body, and simply doesn't allow your brain room for intense self-bullying, I suspect will help. Exercise can also provide longer term mental health benefits from all the good chemicals exercise releases. I started skating again during COVID, and this really helped, because you have to keep all your focus on balancing, otherwise you may fall off! The risk factor really forces the mind to focus. Anything like this, acrobatics, martial arts etc, I suspect will have a similar effect.
Thanks again for posting, and hope you enjoy the rest of your weekend :)