I asked about when you found out because I think that different things help at different stages. For instance, if this is early days, and the revelation of the affair was a huge shock to both of you, I have some suggestions for him (some of it in the video I posted, but also some other helpful and more personal thoughts).
For me it has been important to get the whole truth about the past, but that is perhaps because of the nature of the situation I am in, which I am not going to talk about on here. Finding out and going through the details with my husband demystifies them and allows me to put them to bed. At first, knowing details is painful and 'triggering' but I have found that I can cope much better with things I know than with things I imagine, so I ask the questions. So, OP, have you talked your 'seriously delusional' thoughts through thoroughly and repeatedly with your remorseful and truthful husband?
At first, these discussions with my husband, were incredibly turbulent and were happening very often. After 9 months, the tone has changed and I only have this need to rehash things every few days or every week. If my husband were unable to go through things repeatedly, I would no longer be with him. It has been incredibly hard for him, as he is someone who copes with problems by putting them behind them and looking to the future in a positive way, but he has been able to change his way of dealing with life in order to help me come to terms with his betrayal. Seeing how he has changed, suffered, come to think differently, and got much closer to me, has been the main thing that helps me.
I still have very dark moments, as I mentioned at first, and I am still way too invested in trying to understand his OW. I have a lot of thoughts on this, which again I am not willing to write about here. Definitely, coming to terms with who she is is more important to me than I could have imagined. Before this happened to me I was one of the people who think: "He's the one who betrayed you- she doesn't matter to you". So, OP, have you addressed your feelings about her? Have you figured out what they symbolise in your life? The nature of your husband's affair could easily make my ideas here totally irrelevant to you.
One thing that has helped me immensely is that I have written a series of unsent emails to my husband and to his OW. Some of them are quick messages, some are long discussions which I have thought about quite deeply. They have been a great way to relieve my upset feelings, or to organise my thoughts. These days, I do this for maybe 10 minutes every 3 weeks, but in the first couple of months I did it a lot!
OP, you mention that if you stay with your unfaithful husband people tend to think: "What a mug, how delusional is she?!" Well, I was delusional before I knew that the affair was happening, and I have spent 9 months slowly coming to terms with reality. Now that I am no longer delusional, do I still want to be married to my husband- the real man, not the one in the delusion? For me, the answer has always been: "Yes!". But, that is the question that people who stay in long term marriages have to ask themselves whenever their marriage presents challenges to them.
I think learning to be honest about who you are, and about who the person you choose to spend your life with is, is one of the great challenges, and also one of the great joys, of life. Affairs are one of the things (but by no means the only thing) which can be a sign that the person you are married to is a different person from the one you thought you were married to.