I totally agree. One of the spider monkey's at my gym explained that my basic 'problem' was I can't will myself to deploy more force when I need. I asked how to fix that and he kinda shrugged and just said 'I mean you just pull harder'.
If it is any consolation, I am 3 months into returning to climbing after a 15 year hiatus.... I feel that this "push harder" thing is a feeling that can be learned. Like, when I am moving at near the limits of what I can do, I've been finding that I can just trust my body to do certain things that I don't think it can do (mostly having to do with pulling on very smooth slopers or feeling my feet doing way more pulling than I was previously able to muster).
Simply knowing that I "can" do thing that I did not think I could do has opened up a variety of possibilities.
I dunno if it is any help, but I find that pre-visualization immediately before starting helps immensely.
Hey, I'm about 6 weeks in after a 10 years hiatus, we're in a similar place.
The weird revelation for me was that getting that old strength back is a matter of remembering it, it's memory work. There was this boulder problem I was working on, and I couldn't even stick to the holds, so I used some jugs to ease myself into position, and for the first couple seconds I could barely hang on, but then suddenly it got easier, and it was my muscles remembering how to fire the way they needed to hold on. It's akin maybe to a guitar player who hasn't strummed in a while remembering how to strike a particular chord, it's a little bit of struggle but then it all comes flooding back.
On my first day back I was having a hard time with 5.8, and wondering to myself "Am I a beginner climber again? Do I need to build everything up again from scratch?", but after only 6 weeks I'm pulling 5.12a moves again here and there. I'm pretty impressed with myself, though I'm still a long way from my previous high water mark.
TBH, it was so long ago that I totally expected to be starting from scratch. I am guessing that I'm progressing a lot faster than I might have when I was 20.
I think you're right that it's largely mental, though working carefully on my footwork and other technique has helped with some easy gains. Simply getting that idea about how it works helps-- in my day tpo day life, I generally haven't needed to move like I do when climbing.
Even when I was 20 I couldn't pull 5.12. However, I've been able to successfully do harder "5.11" at my gym. I don't think that will translate to rocks... I could be way, way off base, but the 5.10s in my gym feel a lot like I remember the 5.8s I used to climb back in the day.
So I am super curious how this gym work will convert to climbing in the larger world. Ultimately my goal is to be able to do things like the casual route on the diamond, so I am not aiming to do super hard routes, just committed stuff that takes a lot of skills. I just got a rope and some draws, though, so I'll be seeing how the gym translates to actual rock over the next couple of weeks.
When you have many years of climbing under your belt, and you have a deep well of muscle memory to draw from, pulling at your absolute limit becomes a matter of will.
It's really apparent with someone like Adam Ondra, who has been climbing hard for 20 years. He'll get spit off of a problem a half dozen times and then suck up these elemental forces of nature and crank through it. I mean, he's a genetic freak amongst the upper eschelons of strong, he's more flexible than any top climber today, his ability to climb swiftly with technical precision is the best I've ever seen, but the real X factor that puts him head and shoulders above the rest might just be that he tries harder than anyone else.
You might benefit from reading Eric Horst's Maximum Climbing [0]; it covers precisely this just-pull-harder mentality and provides exercises to unlock it.
That was disheartening.