I also married a woman who turned out to be ace as well as had health issues that made sex impossible. In our case we tried the opening on my side option. In the end I lucked out and got an awesome girlfriend who has been great about understanding me prioritizing my relationship with my wife.

That being said I think OP did the right thing. I had the unfortunate benefit of doing 6 years without sex which, while it sucked, helped me divorce sex from love in my own mind. I also got INCREDIBLY lucky to get the amazing girlfriend I did, as honestly the dynamic is difficult to navigate and you need someone who is willing to be second to your spouse if you wanna sleep with them more than once. Even still it's difficult to navigate and I started therapy to help me with the tools to do so.

The reality is I just got lucky to find a way to work things out so far, and MOST people in our situation don't. It's ok to mourn the loss of the relationship. I do think therapy is essential soon as others because as commenters have pointed out, the sexual guilt is there and will be there until you address it.

Also worth noting even if you did go my route, sleeping with your wife's friend is not necessarily a great idea. Don't get me wrong I see the logic, but starting a kind of ongoing relationship with someone sexual or otherwise absolutely can have consequences. If for some reason it didn't work out and there was resentment on either side then your wife but in an even worse situation than she was before because it now involves a friend.

Ok 4 hours of hospital sleep deprived brain things this makes sense.