What I see:

In everything that you are saying it's as though you are giving your husband a pass here and you're taking on the guilt as if your not trusting him and having lost something by way of love for him is on you and not him. As if you are responsible for the fact that you can't let go of something and that you're in the wrong here.

I have a different take on this. You are doing and feeling what is natural for most people that are betrayed severely. Not only had he cheated on you previously, you have made it clear he is a repeat offender. This 21 yr old girl was not the only event that caused you to lose trust in him and now find it hard to be aroused by him. He did this. You have given him many chances and even now are giving him another and he clearly takes it for granted.

I will be honest, his arrogance in telling you to "just let it go" makes me angry for you to no end. That he thinks he can just say "it's over, let's move on" after having betrayed you so many times is just either complete arrogance or stupidity or both on his end.

In abuse, domestic violence relationships there is a cycle and although cheating may not be considered domestic violence by some, the cycle, I believe is the same. Where the husband, wife or other SO gets caught and goes through a period of what is called the honeymoon period. They are being "all good" to their spouse or mate for a time. That is, until they feel they've got you 100% on board again and then the cycle starts over, whether it be abuse or infidelity the result is the same. I hate to be brutally honest with you but likely this is where you are right now and I cannot lie to you and say I believe that he is a changed man. Nothing in what you've said has given evidence that he's really there for you now. Cheaters that quit will show clear signs of having remorse over it and tbh I don't see that here.

Likely your inner self knows that he's not really there and it's on the surface making it hard for you to be turned on by him anymore because frankly you know he's not all there for you like he wants you to believe.

I know you may not listen to this but have you considered ending the marriage and moving on?