Abbey1989 I have read your sitch, and can relate. It is not good to have your suspicions, and feel lied to, and cheated. I feel like that somedays but can't prove otherwise either. Here is some things to consider:
My first response to a threat of an OW, or OM would be to GAL, DB, 180 and all the other good stuff. I can tell you are already there. I do have to ask though...have you tried detaching? I mean SERIOUS detaching avoid most conversations, move out of the bed, make plans without him (which you already are doing.) Maybe a strong dose of the cold shoulder will make him snap out of it. You have been doing your best, but to no response I think it's time he suffers a little. When he finally asks what's going on you don't even have to allude to OW. Just say I think we are falling apart, I'm just not feeling it. Something about you has changed and I don't like it. If this truly affects him then he will come to you asking how he can change the dynamic. You can then use DB methods to have him work on the marriage, and make HIM do things to win you back. He feels safe having you and OW, it's time to break this fantasy. A big point about DBing is to make yourself so attractive that they feel dumb losing you. At this point he has the better you, and OW. Remember don't say that this is all because of OW, say it just doesn't feel right. He will figure it out eventually.
I know this type of advice is hard, and I wouldn't want to turn you into a WAW. Then again look at all of us men here that have turned ourselves into better men, because our W's went WAW. It really is a wake up call for us men sometimes. Maybe a separation could work. (God all this advice is so not what I would usually give, but after all this time I think it's time to stop going down cheeseless tunnels, that's DB too)
Now all that said consider the following. When I first got married my W was SUPER jealous. She was highly suspicious, and it very nearly ruined us. (Well we both have had issues, so not claiming the high ground here). As a husband it was very stressful to be accused of something I hadn't done. So maybe it is time to get some closure. Consider a way of finding out for sure without revealing your efforts. Perhaps an investigator? Here is the important part, whatever you do, and whatever the outcome, you need to be ready for the results. If they come back negative you HAVE to trust your husband. If they are positive you need to decide whether you can deal with having him near knowing all this, or if it is time to at least move out. As said before, moving out might be the kick he needs to finally walk straight.
