I haven't finished my NEA due to OCD-like thoughts. What do I do, it's due tomorrow.

The final comp sci NEA deadline for my school is tomorrow, and I have no way of finishing it tonight.
My main reason for this is that I've had some heavy symptoms of OCD, which have taken a toll on my studies and revision, and pretty much every type of school work I try to do. I spoke to my school counsellor and I've been put on a CBT programme, so clearly something's up with me.
Now it's come all the way to losing the time to do my coursework and potentially throwing my A-Level.
Should I tell my computing teacher about this and pray that he gives me a one-off extension, or is there no point because it's not an adequate reason, and I'll have to submit it unfinished?

You should absolutely tell your Computing teacher. (In the ideal world, you'd have told them before now as doing so on the day it's due might see suspicious.)
Note that you teachers needs time to mark your NEA, and standardise it amongst other Computing teachers (if any) within the school - to ensure they're all marking to the same standard. Then they need to submit marks to the exam board by their deadline.
So they might not have a lot of flexibility in terms of the school's internal deadline. But it does absolutely zero harm to ask.

I’ll definitely tell him, thank you! The only reason I didn’t was because I didn’t want to use it as an excuse and I didn’t want to really talk about it to any of my teachers as I was afraid they’d see me as being silly or something. However, I don’t really have a choice now unless I want to get a 0. I’m just worried that he won’t care and he’ll brush me off like it’s nothing.

Hopefully he'll be sympathetic and understanding. Do let us know how it goes.

Do you have an OCD diagnosis, if so then this should be part of the reasonable adjustments as you never know when symptoms are going be worse or better, if you don't feel comfortable speaking to them do you have a member of the SEND department that you talk to who would be able to explain the circumstances for you
Hope you get the support that you need and your NEA goes well

Hello, I spoke to my teacher yesterday and we agreed on an extension until Monday. He was very empathetic and understanding, and it turned out he already knew as the counsellor I spoke to a few weeks back had emailed my teachers about this. Although 3 days might not be very long, at least I'll get to finish and be secure in all the sections. Thank you for the advice!

I don't have a diagnosis yet, at least I don't think so. I spoke to my local borough's NHS talking therapies and they did put me into CBT, however they told me that it doesn't start until around 6 months later as the waiting times are so long, so I got given access to a website service for managing my symptoms in the meantime. Not sure if that means I've been diagnosed, but I did speak to my teacher about it and he said he heard about this, which surprised me. I'm pretty sure the counsellor emailed my teachers a few weeks ago when I went to them first, and so I managed to get an extension until Monday.
Looking into the future and reasonable adjustments, perhaps I could get extra time on my A-Level exams, or something like that given that my revision's been affected by this too? I'll ask the safeguarding team about it.
Thank you for the best wishes!