first of all here is a big hold-you-and-let you-cry as much as you want to - hug. I don't think you are just feeling depresssed, i think you really are depressed. It is not clear.. are you still with someone but you just don't love him anymore? Or are you just not with the man you love?
About the abortion.. I had one too, and I am sure you thought about it carefully before you went ahead and I know the medical people would not have let you go ahead if they were not sure you were quite sure and had to do it. You HAVE to forgive yourself for that. I very strongly believe that it is better to set that little soul free rather than accept it into a life where you could not give it what it needed.
Miscarriages are much more common even than we realise. I had 2 and my GP said that most women have miscarriages and that sometimes they don't even realise it was one if they were not TTC. He also said... some souls need only a very little time on earth and we are blessed that they chose us for that little period. You must have something special to be chosen as the "vehicle" for that short while. this may sound soppy nonsense, but I do believe it and if you think that way, it does not hurt so much. NEVER ever however think of it as punishment for your termination. Unless the termination actually damaged you physically there is no reason for it to be a cause.
And you do have your son. Now... Have you always felt like this about your son? How old is he? You obviously do care about him or you would have found a way to leave or give him away or something. You are just so empty at the moment that you have nothing to give...even to your son. But blood is thicker than water and your son will love you whatever.
The very first thing you need to do is make and appointment with you gp TODAY and see if you can get something for the depression and make and appointment with a counsellor. They won't make the bad things go away but will help you cope.
I am so for you about the man you love and his new person...especially if you have to see them both. I am sure people are not laughing at you...you may be surprised to find that there are probably lots of people who sympathise with you.
Feeling alone and unloved is the worst thing...especially in the middle of lots of people. And to be denied the instinctive need to curl up and cry because you need to go to work and keep a straigth face, and because you have a son with demands on you and perhaps a partner(?) you do not love anymore is hard. But people survive!! You need to look after yourself. Talk to friends, or at least to us lot here. Go out for a full body massage. Go for a walk somewhere beautiful. Go to Holland and Barrets and get some Bach rescue remedy and take lots!!! It really helps with shock and depression.
But mostly forgive yourself, love yourself and look after yourself.