Right now, it would seem to you that life seems to be in a big
mess. So it would do you a whole lot of good to just stop for a
short while and think about how you're going to get yourself out of
this mess.
Tackle your problems one at a time. Whatever problems that can be
tackled quickly in the short term should be tackled first. Your
studies, for instance. Since you're already in poly, the best idea
is to simply stay in it. If you believe that poly is tough, believe
me, JC is much, much tougher. The environment there is far more
regimented and you'll have much less time for yourself.
Instead, why not reflect on how you're going to improve on your
studies? If you haven't been doing well, look at why you haven't
been doing well. If it's due to insufficient studying, then you
have to learn to prioritise and balance between all your different
activities.
Next, your relationship. Ask yourself this: is it worth it to be in
this relationship? Can you still see yourself with your boyfriend
five years down the road? If you don't see the relationship heading
anywhere, then I suggest just breaking it up. It's radical, but
let's see it this way: there's no point in causing yourself
continuous pain for the next few years just to delay the inevitable
break-up. I would think that it's far better to break up now, get
sad for a few days, and then get on with your own life.
And please don't think of suicide. It solves nothing, and all it
does is bring pain to your family and friends - the very people
whom you love. Do talk to a school counsellor or someone else you
can trust to help you deal with your problems - there's not much
anyone can really help you on an online forum.
Good luck. 
