Elevation gain limits

Skinning this winter, I’ve noticed I seem to feel great for the first 2,500ft of the day, but after that I very rapidly plummet with a whole-body feeling of exhaustion and a bounding heart (rather than, for example, lactic acid burn or heavy legs). This has happened with a light pack, with a heavy pack, and it’s happened doing laps as well as one long climb (so it’s not just high altitude). It’s happened skinning, booting, and hiking. It seems like it hasn’t improved much despite getting stronger & faster in most other ways.

I’m not quite sure what conclusions to draw. It could be that I just need better hydration & nutrition on the trail. It could also be something missing from my training. Is it important to push elevation gain in training, the way that a marathoner will push the distance of their runs? If I want to do some climb that’s 6,000ft, is it important my training regime includes days of 6,000ft gain?

To be honest I’ve focused so much on duration & intensity that elevation gain has had little attention.