I'm new to this site & have just found your post. LuJay, I sincerely hope that you are hanging on in there.
I too found out my DP had been seeing someone from work. I too found his behaviour just a bit odd, and I pushed and pushed him to talk. Never a great talker he eventually sent me an email at work telling me he'd met someone else and was leaving.
We were engaged to be married at the time.
I honestly thought my world had fallen apart. I picked him up from work to go home and talk. He swore they hadn't slept together but that he wanted to. He said he still loved me and always would. I thought it was just cold feet, but pride kicked in and I told him to leave (I'd learnt from a colleague who'd split from her DP but continued to live together until they sold the house). He was surprised at how strong I was - I told him to sort out putting the house on the market, when he came home a few days later to pick up some things (he stayed at a friend's) I presented him with joint accounts cleaned out of my half of savings, car documents to transfer ownership to me. I told him that the house was half his, but it was no longer his home, and that he could not come and go as he pleased.
3 days after leaving he emailled to tell me about an ex-work colleague of ours he'd bumped into - I told him that he could no longer speak to me about such things, and that our history was just that. Talk to the new woman about that stuff.
I then went to Las Vegas for a week with my sister, over his birthday. I know he had a cr*p day as his family are useless.
All in all I really made him see what he'd thrown away. It was a gamble playing the bitch because I still wanted him and it was hurting me.
That was all 4 years ago.
We went to Relate, and began having a drink after, then meeting for food before sessions, and basically began to date again.
He changed his job so he had no more contact with the woman.
He finally moved back in - to the spare room - but a few months later we moved back together properly.
It was hard for my family and friend's to accept him as they'd seen me so hurt, but they trusted my judgement & wanted me to be happy. This aside they also all liked DP.
However, I believe that we're stronger than ever. He never forgets to tell me how much he loves me and appreciates me. He thought the grass was greener, and found out that was not the case.
We're now trying for a baby, something that he has wanted for a long, long time. I worry that some people may think we're trying to "paper over cracks", but I can honestly say that is not the case. We do not take each for granted as we had a taste of what life without each other would be like, and we didn't like it.
Councelling is a definite yes, being honest is a definite yes, and taking some time out from each other to lick your wounds is a definite yes.
I wish you every luck and happiness. You really sound like you deserve it. It can be achieved with your DH, but I wish you well whatever path you choose, xx