Hi OP, 18 months is nothing in the grand scheme of things - they say that it takes five years to get over a major relationship and in my experience this has proven to be true.
You may feel that you are no further on but you will have made more progress than you realise.
The pain seems unbearable, but you are learning the most important lessons of life right now and you will come out of this a better, stronger, wiser and more compassionate person.
The pain WILL pass. Unfortunately it just takes time.
As far as is possible, cut him out of your life. If you must interact with him because of the DCs, try to keep communication brief and don't let him have the satisfaction of knowing how you feel.
Try to act happy (at least in his presence). Not to play games but because sometimes you have to force the body to act, then the mind follows. Scientists have conducted experiments to prove that this works.
Most of all, I echo what a PP said - do the things you love as often as possible. Go out and meet new people, new families and single mums. Join a single mums' group perhaps?
What helped me when I was going through this were the following thoughts - they are cliches but true:
There is a reason for everything; if you are supposed to be together, you will be at some point in the future. If not, it was not meant to be.
Just because you love someone, doesn't mean you have to be involved with them. Love is not a bandage to cover wounds. (Hugh Elliott)
God has a plan for your little life.
It will take time, OP, but one thing is certain: you WILL get over it.
x x x x