Op, have you thought that your need to create a public and large and dramatic ending to this situation is to do with making this part of your life, that has meant so much to you, and changed you, be more "real"?
This affair was/is real, it's happened, it's effected your life, it's been a big part of who you are for 2 years, yet if you quietly end it, it'll be like it never happened, particularly if MM never tells and just carries on with his life. It could be that it seems insane that something so significant to your life is something that will just end with no one knowing about it.
You aren't planning to tell MM's dw because she has a right to know, or because it's the only way to end it, or even because you want to punish mm, but in order to force everyone to know about something that's been so important to you.
However, that instinct, for the dramatic ending, will steal any possible control you could have over the next few years. If you plan to stay with your DH for a couple of years longer, to try to get yourself into a position where you can support your dcs alone, to have the time and space to sort out your life and your dcs lives, then you need to accept this is part of your life you just park for now.
Just tell mm it's over, then mean it in your head, no bridge burning in a dramatic way, no big emotional "but it's wrong", just a simple "I've decided I don't want to be on a relationship with you anymore." Don't feel you should spare his feelings, in fact I'd aim for hurtful and nasty (because he at least deserves that!) - it'll reduce the chance he'll chase you if he thinks that you genuinely don't want him, not that you do want him, just not the situation.
Then take all the energy and effort this affair has required to keep it going for 2 years and put it into your new life plan. Ime, people in shit situations who have affairs end up with the affair keeping them in the shit situation, it takes too much headspace for you to focus on improving/getting out of a crap situation, by providing excitement and romance and interest, it masks that those are lacking from your life.
Years ago, before I married DH, I had a very dull job. There was a very gorgeous and funny married man there, who I'd chat too, play silly games over the intranet with (along with others) and generally have fun with. Nothing happened, and I don't think either of us would have been interested in an affair. He was head hunted to a competitor, and within a month, I'd updated my cv and was looking to go elsewhere. It wasn't that I couldn't cope without him, it was without him there, I realised how dull my job was and that I could do better.
With MM in your life, you might finally realise you can and should improve your life.
(Sorry for mammoth post!)