I think I'm screwed (UCAS application problems)

What's your Alevels?

Chemistry, biology and AS history (yes, I know I don't have 3 full a levels, its due to medical and mental health issues that forced me to drop a2 history to AS)

Chemistry, biology and AS history (yes, I know I don't have 3 full a levels, its due to medical and mental health issues that forced me to drop a2 history to AS)

You don't have maths or even 3 full A-levels. If you want to do compsci, the best thing would be to take a gap year and complete the full Maths course in the year.

Take a year out, figure out what you want to do, and then take any necessary A Levels. That will save you a lot of stress and expense, which is the likely result of signing up to a course for the wrong reasons, struggling, dropping out, and starting all over again.

I can't do that, they're ditching the old spec of Maths for the linear one, the new a2 exams won't be avaliable until 2019. I already found a few unis that would take me with 2 a levels and my circumstances

I really don't want to have to take a gap year, I can't take more Alevels due to the fact that they are linear and so apparently you can't take AS and A2 in the same year.

In the case of maths that's not true. The new spec is up for first teaching in autumn 2017, so schools and colleges will still be doing a final round of old spec exams in summer 2018 for their A2 students. And it's only maths that you would really need if you decide on computer science (and that combination would open up many other courses regardless).

Gap years have an unfair reputation as wasted years, but a year out is only wasted if you want it to be. Honestly, it's no big deal and people take time out all the time. Nobody worth listening to will judge you for it.

I just didn't want to take a gap year, I'm not capable of learning all of that Maths in one year and I know I would do nothing with a gap year.

Take your time

Look for courses that are still open on UCAS course search (wait until the middle of the week when universities have been told they can close courses). Without maths or comp sci A level then it would be worth looking at courses with a foundation year - they'll cover the maths you need (instead of covering all the maths of an A level) and are likely to be more welcoming to someone with only 2 A levels and an AS (as well as late applicants). Call any universities you're interested in to double check that they're accepting late applications (and maybe explain a little about your academic situation - they wont lie about your chances and by checking you wont wast any of your 5 choices)

Once you've got a shortlist of courses THEN write your PS. At that point you'll know what it is about the course (content, careers, specifics) that is attracting you and will have more to write about. It wont take long to write your PS.

Have you had any help from your school/college? Are they willing to provide a reference?