It's a slippery slope to climb, keep it up if you can. I too find it harder to find the energy and will to fight as I get older, I think there is resilience in youth.
I try to think that the measure of my achievement is not so much what I do, but how many barriers I had to overcome to do it.
For one person a PhD may be less difficult than for another to get out of bed every day.
Be kind to yourself as much as you can, YOU DESERVE IT!