I couldn't read without posting.
Is there a way for you and your husband to get away together, with just the two of you, for the weekend? You two have some serious things to talk about, and you've been open and honest enough with each other up until this point, don't stop now.
You need to work this out and put an end to the causes of the problems.
Many men have fantasies about bringing in a second woman/third person to play around with in the bedroom in order to share that with their wives and to spice things up a bit.
It's a different thing to ACT on it. That changes things. We aren't Vulcans, when we do things that are intense, we have emotional reactions to them, envy, jealousy, etc.
I understand why you did what you did, I imagine that it's all part of being THAT close with someone, to share EVERYTHING with them, that's part of being a best friend and being close to someone.
But, the institution of marriage is there for a reason, to protect you while you are pregnant and nursing children and to protect your children. When (in my opinion) you start messing with that institution, monogamy, etc, then you really get problems because there are children involved.
If you still love your man, and it sure sounds like you two really love each other, I strongly recommend that you get away together and work this out, for you two and for your children.
Maybe even renew your vows? Re-affirm your commitment?
Is that the route you were wanting to go?
I'm so sorry that you're in troubled spot right now, but I hope that you're able to fix this with your husband.