Too late for accommodations? Posted here as OP is not on SE:
Ok I have a dilemma. I am a university student. I was re-diagnosed late with ADHD due to a misdiagnosis of severe depression and anxiety, I have struggled at university often having submited assignments late with careless mistakes. 

Now I know that the standard concessions given to someone with my condition are extensions plus additional support someone to help break down the assignments and help with structuring my arguments through providing strategies to organise my work. 

My problem is the majority of my low marks and fails were caused by penalties for late submission 5% per work day. I failed multiple assignments being hit for the maximum penalty of 50% (10 working days). 

Now that I have my late diagnosis, should I contest those results and ask the penalties be withdrawn or tempered? Or should I just go dig a hole and bury myself because without that degree that is about all I have left to do. ‍♂️
Go talk to someone at the university who has some responsibility in such matters. There is often a specialized office for it. Discuss the situation there and ask for advice. They have knowledge that few of us here have and can make suggestions few of us might think of. 

But it isn't necessarily a question of contesting results. Find out your broader options before you choose any particular one. There may be some combination of things that can be made to work for you. 

You aren't alone in such things, and universities short be adept at handling them.