Curious about your breakdown between heavy lifting vs. cardio. I find that I can't do more than 4x/week of heavy lifting, so I've been trying to run (or something else) on the remaining 3 days.
If I show up at the gym and really feel lacking, I do a different muscle group. If I show up to the gym and I'm supposed to do bench, but I'm hurting - squat instead. Squats struggling? Do a pull up workout (I'm trying to get to 500 pull ups in a single workout). Whole upper body hurt? Push the sled around. Or do a full body weight workout which acts more as an active recovery.
There are a limitless amount of things that can be done.
To quote Mark Rippetoe (it's likely in the article somewhere on his site, but I primarily consume his advice via podcast), "you don't get strong in the gym (as you mainly tear muscle in the gym) - you get strong when you sleep and recover".
Have you tried just upping your sleep?