I'm a 3rd year astrophysics and cosmology student at Lancaster Uni and I started year 12 doing maths, further maths, physics, econ and psychology and found I was achieving mid-level grades due to the number of subjects I was doing and the FM course being too challenging for me. What helped me make my decision to drop psychology and FM was finding out what I wanted to study at uni and seeing what a-levels were necessary / what I should be putting my effort into.
I think in terms of your decision, as you have already seen what you would like to do in the futrure and having FM as a completed a-level is something you would like to do then you either start prioritising FM as a way to show your teachers you can do well or you explore courses in the fiel you want to study that don't require it. Saying that, I belive most courses don't require FM so if it is something that is causing you stress and being under a lot of pressure then it might not be the best idea to continue with it and it may be best for you to ficus on your other subjects.
You mentioned being interested in finance as well as comp sci and I think researching degree courses in either field and reading their course outline would help you see what you would prefer to do. Some unis go into a fair bit of detail so take advantage of that and see which ones take your fancy / how it compares to the other subject. If you've already done that and are still unsure then look at what future career you might want to have: if you find something you prefer in one, then you can pursue that. However, having something like comp sci would allow to to work within finance as well, so looking at skill sets that you gain by doing a particular subject could overlap with the other, so don't worry so much about making this decision / feeling overwhelmed by it as there are quite a few interchangeable skills that allow you to have a career in either if you just study one of the subjects.
I hope this helps and if you have any other queries, feel free to ask!
--Arya (Lancaster University Student Ambassador)