I don't know what to do anymore

I'm in year 12 and I wanted to do biology, geography, psychology and sociology but because of my C grade at GCSE I was unable to get onto the biology course. I knew since last summer that at Uni I wanted to study marine biology (I know it's super hard but I would have worked for it) and because I didn't do biology I chose history. I am so unhappy now with how school is going; for the last 2/3 months I have been trying to drop history because I am so far behind and it's too much for me. On top of all this I am currently on a U in psychology and my head of year is trying to make me drop it now. I am so stuck on what to do because I want to carry on geography and psychology next year. I feel like my school is no help and they don't listen to me and because of how things are going I don't want to go to university anymore and I feel like I am going to go nowhere in life. Pretty much every night when I get home I start to revise and then break down because it's too much work on top of homework and then I'm not in the right frame of mind to do anymore and I just give up. I have now realised that I should have taken BTECs but it is too late for that and I do not want to resist the year.