When there is no way out... you start looking for a way in

It is a long story but I wish to present it in 10 lines and I’ll try! Just 5-6 years ago I had a happy marriage with 2 beautiful children.
Then SHE appeared, and turned our life into HELL, but, as it seems, adultery is something normal these days.
The triangle is obvious!
I can’t see a way out of the situation, so obviously I have to seek a way in. But how and where?
My husband doesn’t feel well lately. He doesn’t want to abandon me and he also has twinges of conscience, but he doesn’t have the strength to leave HER too.
After all, 5-6 years is quite a long time for a relationship. In these years I’ve changed both psychically and physically. I became stronger, more confident in myself, etc. This change was noticed.
Anyway or another HE didn’t put an end to his relationship – I understand him! I myself feel bad when I see his condition. I even don’t know on what stage their affair is and I don’t want to know.
The only thing I know for sure is that I need somebody beside me, and obviously that’s not him!
How can I help him and what is the best thing to be done for both of us? I gave him a great while to live through his relationship, but this didn’t sort matters out. We have so much that connects us…. and 20 years of marriage!
IS THERE SOMEBODY WHO CAN SHOW ME THE WAY IN?