I am sorry you have been dealt such poor behavior by your DP.
I am going to be another one stating that he has found someone else, but he is not going to be honest about it with you, no man will ever admit to this.
This has to be hidden from you at all cost, and later you will learn how he hid the OW until he was sure of his choice.
So right now its either her or you.
OW may even have given him ultimatum on this with him. They normally do.
This is why he has told you that he is not happy, needs to move out and be on his own, which is bullshit feed as he does not want to be portrait as the bad partner who left kids and wife for OW, its all about holding ego and face for the outside world, with no care for you or his DC. Now he is wanting to test drive the OW first prior to taking final decisions on you or her, he will tell you that he is not happy, or in short re write history with claims that just dont fit in with reality.
There is the cheaters script you can find by googling, and then there is the Chump Lady, have a look at both. Also find out as much as you can on what you may be entitled to regarding benefits as previous posters have advised.
If he comes back, it will be because the OW just wasnt as much fun as he may have thought. And if you take him back, you will never feel secure in the relationship and always second guessing yourself. Your relationship is effectively over so do not fall into the pit of hoping you can change the scenario that is unfolding.