You rightly point out that you need to "find support and I don't know how".
If you look to others to support you, love you, cheer you up, make you feel loved, or special, you are looking at the wrong place. Others will ALWAYS let us down in a big or small way.. So, what do we do?
We look to OURSELVES. Every person has enough inner strength and abilities to keep him safe and well cared for, if only one would look to find them.
I will give you an example:
You say you are a full time student on a restricted budget. Don't you think this alone speaks volumes about your inner strength to survive on your own, to cope with your responsibilities and get on with your life all by yourself? Don't say "No", you are already doing it!
You say: "this lump in my throat and the constriction in my chest is just not healthy". Isn't this an indication of maturity on your part and a strong sense of self preservation when you recognize that he is hurting you in an UNhealthy way?
So, if you look deep inside you, you will find other qualities you possess in abundance which you have, perhaps, left unnoticed and not utilized until now.
If I am right, you will again find the inner strength to do what you already know is necessary with this irresponsible guy. You have youth, you have strength, you have brains. These are powerfull resources that will see you through this difficult period of your life. Look also for new friends, new hobbies, new activites and you will find lots of things to keep you occupied and think less of him. Time will eventually heal you and set you free.
Good luck and God bless you