Re: Affairs (Physical and emotional)
I agree with most PPs. You cannot do it alone, he has to be on board. There have been terrible times in my marriage, but both DH and I have wanted to work it out, so while it's been difficult, it worked. If he's not really there, it's out of your hands. You deserve to be happy and be with someone who respects you.
If you do get serious about leaving, he may come around. He might have just gotten used to taking you for granted. Seeing you getting ready to leave might wake him up!
If not you know that leaving is best for you and for your boys. You need to set good examples for them, and him cheating and you allowing it will not do that for them.
Good luck! and if it helps you to talk to a licensed therapist, I definitely concur with that! If you can talk to a therapist you'll know you explored every option and will be in a better place emotionally to handle what comes next.