There are a lot of variables here and realistically I don't expect concrete answers without taking steps that I, frankly, probably won't take. What I'm probably looking for is validation or similar experiences.

I wrecked my bike pretty bad last year which resulted in almost zero riding, or much exercise at all for that matter, for about eight months. A predictable combination of 45 y/o male existential crisis/self loathing/depression took it's toll over those months and I gained about 30lb without even trying. That's my super power these days...gaining weight while eating rabbit food.

Since April, I've been doing 2-3 rides per week, at least one of them "hard" (for me that's >15mi of mountain biking with over 3000ft of climbing and with my HR averaging 160 or higher), and lifting 2-3 times per week. At first, I was doing as much volume as I could as long as it wasn't impeding my shoulder and back rehab but now that my shoulder is better, I've upped the weight and have been doing ~60% of max, 3 work sets with max reps and then accessory lifts that focus on shoulder stability. I still can't do much that loads my back, so leg work has mostly been intervals on the bike, stair climbing and leg extensions and curls with lowish weight and ~20 reps. Each workout starts with a 40min warmup on a bike at a fairly low HR of ~130. I've been trying to maintain a calorie deficit so that I'm losing 1-2lb per week which has mostly been going well.

The other variable here are severe seasonal allergies.

I ride with HR and power and have noticed this month that at a given HR, say 165, my power output is far less than what it should be and seems to be getting worse with each ride. I took nearly the whole week off last week. Easy ride on Friday, heavy lifting Sunday night (chest and back) with no accessory movements and then I rode yesterday and felt like absolute shit. Granted it was 40° hotter than my last ride but I can typically hold about ~260 watts at ~165BPM for the 35min road climb to the trail head. Power has been dropping over the last month by a few percent each ride and yesterday I was at ~220watts at ~165bpm over the first 2/3 so I backed off for the last third because I had a certain ride planned for the day. On my second trip up the hill, I stopped for a minute, which isn't a thing I'd normally do. My output dropped to ~180 with my HR creeping up to 165 and I think it hurt more mentally than physically.

These past 2-3 weeks, I've just been amazingly tired all the time and can't seem to snap out of it.

With all that, what's the right path forward here? I don't feel overtrained in the way that I know I have been a few times in the past but at the same time, I realize that as I age my ability to recover is slowing down. I still have about 15lb to go before I feel comfortable (and then 15 after that to get to an adjusted "ideal" weight) but feel like I'm just not capable of pushing past this sticking point that I've hit. I've not hit the point where hard physical work releases endorphins like it has in the past, this just always feels like a trudge.