The first thing to do is take a deep breath - as a big worrier myself, I understand how hard it can be to keep calm in situations like this!
I also understand that you might have lost belief in yourself for biology, but I do think one of the ‘main’ 3 sciences is a good idea, considering your future aspirations. Are your doubts definite? You may just be worrying and overthinking, is there any way you could change later on (a couple weeks into the term) so you at least get to try it?
If you do choose to replace it, a facilitating subject is probably a good idea if you aren’t really sure exactly what to replace it with.
Overall, the main advice I’ve heard for making a decision is to put all of the options into a hat and choose one at random. If you’re happy with what comes out, great, but if you feel a bit disappointed, then you know that it might not be the right choice for you!
I wish you luck in making this decision. Let me know if I can help any more.