I strongly agree with the suggestion to do couples counseling. ADHD is a sonofabitch that makes us work twice as hard just to get half a far as someone without ADHD. There's legitimately only so much we can do, despite our strong feelings of having the squandered opportunity to do more. NT people will never be able to understand what it's like to have ADHD, just like we'll never be able to understand what it's like to be NT. A good therapist should help to bridge that gap.
And I sympathize with you greatly. I've had many relationships of all different sorts suddenly implode because of my ADHD throughout life. I hate that helpless black-hole feeling. I'm extremely lucky in the fact that my wife also has ADHD, but I definitely still feel that extremely uncomfortable strain at times. A thing that has helped me is to recognize that those relationships have never truly ended while I still carry the memories and the way they've changed me. It still hurts like a motherfucker, but it hurts less.
I know it hurts a whole lot as I can't keep myself from legitimately crying right now just remembering how I can relate, but I hope that helps to let you know that you're not alone. Keep fighting the good fight and I'll try to do the same.
Mentioning covid is actually spot on for me. I can only speak of my experience, but I seriously doubt my experience with ADHD becoming dramatically worse during covid lockdown and never really recovering is unique to me.