Don't give up! I wanted to do that course but it did not start at the time I needed it to.
Doing an OU degree is one of the hardest things you will ever do.
Two things, firstly do not give up as you have come so far already and it would be silly to give up now. Is the assignment substitutable? How much have you done so far? When is it due in? Spoken to tutor yet?
It feels so bad when you have a tma deadline, lile you couldn't possibly cope. But when you click submit...ahhhh....that feeling is awesome.
Secondly, I know all of this because I have just finished my degree, last week. I really do understand, I have been there, for 5 years full of tears.
But now I feel so proud of myself. I am planning what to wear to my graduation this year. I cannot wait! It still feels a bit weird but I feel less stressed already but already miss it.
You are doing a great thing, anything is negotiable too, in terms of deadlines etc. Beleive me I know 
Where are you up to with it now?