Hi someone else here to let you know that it will get better. I think sometimes it takes so much strength to make everything OK for everyone else that when you are on your own you just collapse.
I know for me Friday nights were always the worst. I was just shattered and would always end up sobbing to myself once DD was in bed. The thing is though, I think that is probably part of the healing process - grieving and letting it all out.
As Lying said, it's important to look after yourself, even if it's just making yourself some nice food and having a glass of wine once the kids are in bed.
Actually I was feeling a bit sad myself tonight, but in a way that may give you some hope.
I split up with DD's dad almost 10 years ago when she was also 2. It wasn't as long a relationship as yours and no OW involved, but I was still heartbroken. I would be fine at work then get on the train to go home and just find myself crying.
It was really tough for a while and Ex-P and I had a terrible relationship for quite some time. Anyway, it's her birthday tomorrow and this evening, the three of us, plus one of her friends all went out for dinner together and had a lovely evening.
DD has a good relationship with her dad and he and I get on fine. The reason I was sad was really because I was feeling sorry for him. He regrets terribly what happened and wishes he hadn't left when he did, but it's too late for me to go back.
Although there are lots of things I like and admire about him, I know that I am happier without him, and I know that DD is fine too. More than fine!
I am not in a relationship at the moment (which still does make me sad sometimes), but I am really happy with my life in general, and we have found a new way to be a family.
No one knows what will happen in life, but things will get better and you won't always feel this sad. It is a cliche, but time really does heal.
Family and friends - and of course your DC - do make things better, and gradually you will find that all the positive things in your life, start to wear away the sadness...
I hope you have something lovely planned for the weekend.