The kinds of fears you have about failure, mediocrity, getting stuck, etc. I think are normal even for people who have excellent mental health -- I experience them daily. We work in a field in which continuous learning and progress are expected, and real learning and growth are necessarily uncomfortable. You have to spend a lot of time scratching your head and feeling like you're confused and not good at something before you can get good at it, and that can be very discouraging.
So for me (and I'm sure it'll be the case for you too), the challenge is to find a way to work with what you have rather than somehow expecting your mindset to change. It'll be great if and when you do feel like you've found some peace of mind and stability, but until that day comes, you can and should do what you can with what you have. Structure your work and life around what you can do, not what you can't do. It seems like you've already taken some steps to do that, and that's something to see as a strength rather than a weakness.
The nice thing about work (as long as it's not on a deadline) is that it will wait for you to return to it whenever you're ready. You can retreat from it for a few days or weeks, you can fall down into a hole, and when you come back, you won't have lost anything as far as your work is concerned -- you can pick up where you left off. The challenge is that progress can feel SLOW because of a lack of continuity and momentum. This is something that must be accepted if we are going to practice in a dedicated way, even if it makes us uncomfortable.
It's not simply a matter of 'toughing it out', though. If you try something for a couple days or a couple weeks, and you can handle the strain and see it as something temporary and bearable, it is good to stick to a plan and see where it takes you. But if you feel so much resistance that it's all you can think about, it's OK (and good) to try to switch tactics. You keep doing this (maybe for a long time), until you find something that sticks even a little, and then from there you can build on it. If you fall, you can come back to it. It's a spiral nature and even if it seems like you have gone back to square one, you are actually rising a little bit with each new experience.