keepingup Easy tiger! Slow it all down. Stop. Breathe and work out what YOU want.
My money says OM is more than likely a catalyst. he may be more, but he may not be. He represents the problem you have in that your marriage is dead. If it weren't, you wouldn't have fallen into this. You are banking on this OM being the answer to all your prayers. he isn't. YOU are.
You may be also terrified of being on your own, it does come across that way. You can easily be on your own, and tbh, you need to be, for a while.
You say you should break it all off with OM if you want to stay with DH, tbh, you need to break it all off with him anyway, for much more than 3m, for good. Otherwise you are not dealing with the issues, merely waiting till you can resume the non-relationship.
This is a waste of your time at the moment, it's disloyal to your DH; a man you say is a good one. Ask yourself what your DC would think if you try and explain what's going on at the moment? How could you think they would approve of this.
Integrity is what is needed. If your relationship with DH is no longer viable, then bring it to a close, properly and with no external interference. Focus on making sure your DC are happy and OK with the break and get yourselves into a routine that works for all of you.
THEN, when you have grieved for your marriage, worked out who you are now, and understand what makes you tick, THEN you can look at starting over with another man.
If you had no DC, I'd still give the same advice, but with them in the picture, it's crucial that you help them through this.