Could it be that there's a physical reason which has an impact on your feelings? I do get the endorphin high, but really only when I've done really testing cardio, or a run outside (I'm 67). If I overwork, I do get that exhausted headachey feeling - a prompt to drink as much water as I can manage!
Maybe you're just tired/exhausted more generally? It might be good to get a doctor's check up, although the NHS doesn't really deal well with this sort of thing, unfortunately.
Can you add in exercise you enjoy? So you do something 6 out 7 days a week? ie one complete rest day, but an extra something physical each day, more than your ordinary walking that is.
Have a think about diet - pre- and post-workout.
It could be that it's the programme itself. I find the "senior" exercise classes that are run in my town are vastly inadequate for me, but I've been active my whole life (trained dancer, aerobics in the 80s, now getting strong AF weightlifting more than my body weight). Maybe you need more of a challenge?
But it could just be a blood sugar drop ...
I find if I don't exercise I feel low.