Hang in there praying! I am willing to bet that 99% of your conversations with H have nothing to do with R. Most are just "fun", how was your day, small talk and he probably talks more than you. I'll bet you validate him just as much or more than 21 year old does! It is the way he is using His mind that makes him feel "bad" when he talks to you and good when talking to her. I will bet you are a better partner to him now then you have been in years but it is he who refuses to see this. If he let himself see this than he would need to change his behavior and that can't be allowed to happen!
I have been a husband for 20 years and a dad for 18. For most of that time I worked while my wife had the option to either work or be a stay at home mom. I put her and my kids needs ahead of my own for all that time because that is the right thing to do! I'm not angry or upset about it. I'm not going to blame my wife and suddenly become selfish and act like a teenager! That is his CHOICE, nothing more and even if it was true he has been doing EVERYTHING for others for years, it doesn't make it right for him to throw away his marriage and hurt the people who love him most (a count on him).
I too had a bad night. Having money problems caused mainly by wife (she won't admit this) and the stress of that, my marriage, my kids, my job is weighing heavy. Hang in there. We'll both get through this. Hopefully our S's will come around in time. All we can do is try to keep DBing and pray our brains out!
