I am really sorry to hear you have felt so depressed and hope you have the support to kickstart and get a happier mindset out of it. Regaining laughter after depression can be like flicking a switch to start the engine and who knows with practice to change what you can achieve will be amazing. Then your future will start to look a little brighter. Maybe on your life journey you have lost the ability to value your self worth? Maybe you are just stuck on the record on negative statements and outlook? I don't know what your original issues were, all I can say is if you can find inner peace and happiness you are are able to climb out of any dark cave. Some of that comes from just living life without feeling judged by others. Have the courage just to be you, to just 'be'. You can strive all of your life for the best things, best clothes, a big house, nice cars, best grades, but if you don't have peace and happiness and contentment it will all be wasted. You can live in a cardboard box and have the head space of a millionaire, be really happy and content just because you can look at life's positives not negatives.
Negative thinking is a hard concept to unlearn when you are so introspective and the whole 'performance' issue has been about you (yes it's the all about me, me, me) That has taken so many years to become entrenched in you - important yes but you can change it. It is also linked with Imposter Syndrome (I will never be good enough syndrome) By all means apply your standards of 'not good enough grades' to whichever Uni or training course you want but get your head into a better outlook and know what you want before you make sweeping decisions. You have your whole life ahead of you to study, nothing is wasted.
Life isn't so much about an arbitrary list of grades, it is about how you take those grade numbers and run with them. It's not what you have that matters it is what you do with it. You clearly have a talent to produce pieces of artwork that others are interested in. So when I say about the 'me me me' part you have to let go of your own narrow prism of your own likes and dislikes, your viewpoint of your own artwork. All that matters is that someone somewhere likes it, can identify with it and your piece of art work strikes a memory moment or chord for them to want that which you have created. So in many respects you are the irrelevance - you create something, step back and put a price tag on it. When others like what you do you have to understand that is all that matters. It's about them and not you. You can create a product (even if you don't like it) that others like and they will love what you have created. Be comfortable with that and use that to build the inner building blocks - keep saying 'I am doing ok' Monitor negative statements in your head and change that narrative for a positive one.
Have a look at the MIND website and use their self help CBT style information. Have a look at the Maudsley Charity supported by the Arts Network. Although the charity is based in Lewisham they might help to signpost you elsewhere with similar support networks.
Keep going and never look back