How do you calm stress levels when things like this are going on?

So basically i'm in my second year of college and i want to study maths at university. I want to go to Leeds and i need an A in maths plus another A and a B to do this. The thing is is that i've not done very well in my recent exams so it looks more likely that i'm going to get a B. I want to try my hardest to get an A but i don't know where to start. My teachers really aren't supportive, i don't think that they believe that i can do it even though i know i can. Their negativity is reflecting my attitude and it's really bringing me down. I haven't told my parents yet what i got in my results because i feel like then they won't believe i can do it too. Alongside this i'm struggling with relationship problems with my ex boyfriend and this girl he's now getting into. I've learnt that both her and him want to go to Leeds also and it's adding more and more pressure on me to get in aswell. I can't get over my ex boyfriend and i'm facing the most important exams of my life in literally 2 months which could make or break getting into uni. My other A and B grades are fine, i just might narrowly miss the A in maths and if i can't take my mind off other problems how am i supposed to concentrate and get the grade i need? I don't know what to do, i'm running out of time and starting to panic. Any suggestions would be helpful.