I don't understand how someone can love someone yet lie to them and cheat on them.

I gotta say - in regards to how you've treated your wife, yeah, you failed. You should feel guilty for it. Don't blame an illness for treating your wife that badly.

Does your wife know yet? If so, ask her what she wants to do. If she wants to end the relationship then you'll have to accept that.

You can't maintain two relationships unless both other partners are polyamorous. Even then there can be strains. You're being really disrespectful to both of them. Your wife doesn't deserve to be treated like your needs go above hers - marriages are supposed to be partnerships. She doesn't deserve to be made to feel like she is not worth it. Your mistress doesn't deserve to be led on, if you claim you love your wife more than her. She doesn't deserve being used to feed your ego, to boost your own self-esteem.

Neither of them deserve to be lied to and believe that you love them more than the other.

Do you attend therapy? Solo therapy? It sounds like you need that along with the marriage counselling.

I honestly don't have anything more constructive for you outside of sharing how it looks like you're treating both of these women. You made these decisions, willingly, and no amount of guilt, regret, or self-despair can make up for how terribly you've treated both of them.

If you aren't happy enough with your wife to remain loyal to her, why stay in the marriage? You could move on from one another, and both have the opportunity to find happiness. If she doesn't want that and wants to try to work with you, then you need to end the relationship (and friendship) with your mistress.