Be kind to yourself. Realise that people don't fuck you over because you are flawed, they do it because they are flawed. I don't think anyone here would suggest that what this guy did is OK. It's not OK to date someone for three months and tell them you want a relationship and then just suddenly drop off the face of the planet. That is fully lame and a dirtbag thing to do. It completely understandable that you should be upset. To be honest I think sending a message out on Facebook like you did is understandable as well

Sure, all his friends probably think you're a fruit loop but who gives a fuck? I bet it felt good.
Letting this totally freak you out and effect your grades and put you in hospital and giving you a life attitude of "nobody will ever love me" is not reasonable however and to be honest, it's a little self-indulgent. It's OK though, maybe you learnt this kind of self-indulgent, dramatic, end of the world kind of behaviour from your parents and you don't have a role model for productive behaviour and coping skills. I can relate to that. So where are you going to learn this stuff?
Sadly, not from Bluelight. You can learn alot here but I think the stuff you need to learn will be learned from observing people coping and behaving honourably and doing the right thing in their day to day life. Look around you, who do you know who is kind and good and basically enjoying their life? What can you learn from them? I don't mean by going up to them and asking them to teach you stuff either, just by quietly observing them an listening to them what can you learn?
What do people do when they feel inadequate? Personally, I sulk for a day or two, then I see if I can do anything to improve the situation, then if I can't I move on and focus on areas of my life where I can be adequate.
What do people do when they feel they have no control? Let go
What do people do when they feel that other people are harming them? Tell that person what is wrong, give them one chance to fix it (or work on fixing it) then remove them from your life if the situation cannot be resolved. It feels bad removing people from your life sometimes, but it saves you alot of time and angst you don't need.
What do people do when they feel they have fucked up? Apologise, make amends - this applies even if you are only apologising and making amends to yourself.
And yes, you might want to think about doing some volunteer work at some point.