I had a particularly hard strength training session on the turbo today (Z4/5) and I cracked half way through,
Gee....I hate to be the one to break this to you, but numerous psychological studies have shown that once you "crack" during a hard strength training session, you can never really recover fully. Your psyche has been permanently damaged. Oh, sure.....you can TRY to HTFU.....you can still "push" yourself for a few moments at times...but it will just never work. Many professional and amateur athletes have vainly persisted in their futile attempts to regain some sort of minimal "fitness" after such an incident, but they always remain just a pathetic shadow of their former selves. "Cracking" such as you have done was the origin of the phrase "You just can't come back after that!"
Don't despair. Your athletic career is definitely over, but there is still a whole world of alcoholic beverages to explore and comfy chairs to sample and enjoy.
For God's sake, just don't subject yourself, and especially your family and friends, to the sorry spectacle if an "attempted comeback." This is for your own self respect. Just let it go. It was a period of your life that is now over and you can savor any good memories you may have. Know when to move on.