I had skipped over this thread until now because of how the question was posed.
However, seeing it now I have two things for the OP:
- The idea that you were “overtraining” previously and your body just didn’t have the recovery ability is exceedingly unlikely to be correct. For example, I am 47 years old and do not have above-average genetics and even if what you were doing before is much more than now, I routinely totally train you into the ground, and what I am doing is not overtraining.
Not because I am better, but because it is perfectly ordinary for the body to be able to handle and to well-utlize more than what you are doing. And yes, to “recover” from it.
You have bought into some BS.
Don’t feel overly bad: I had undertrained in terms of volume for years because of mistaken such ideas as well.
- As already mentioned, this business of how you do only one exercise per bodypart and only 2 or 3 sets of it for 7 reps each and then you are just plumb tuckered out and anything more would be “overtraining” is just the purest BS.
Work harder.
I do see on this last page that you have an open mind to increasing your volume.
As an example of a legitimate case of having reached a point where temporary strength loss means that more work isn’t called for: About 10 years ago, I was doing two sets each of probably 5 back exercises. The gym had just relatively recently gotten in a Hammer Strength Iso Row machine, and I’d always – maybe for a 6 month period or longer – had it as the last of my back exercises.
I thought I was training it from 60-90% 1RM, the 1RM having been estimated from performances in workouts.
I wasn’t making too much progress on it.
Finally I decided to take a fresh actual 1RM.
I was way stronger on it than what I had thought. When I was training at what I thought was 60% 1RM and getting two sets of 9 reps, I really only had (I forget now exactly) something like 35% 1RM on it.
In other words, I was doing the exercise at a point where I had already worked so much that my strength was barely more than half what it would be when fresh.
That was grinding it into the ground and is an example of what not to do.
But strength dropping – due to work done – to where you can get say 6 reps where fresh you could get 10, or 3 reps where if fresh you could get 5, is a perfectly reasonable thing when doing multiple exercises for a bodypart.
Or alternately, if wanting to do only one exercise for a bodypart, then the idea of the sets preceding the final set is NOT to have that last set be done in a much weakened state, but to be in good shape for that set and do strong work on it.
So for example, let’s say you want to do somewhere around 25 total reps and you happen to like this number 7 that you’ve mentioned for reps.
Don’t do it as, for example, 10 (all that you can get with the weight), 7, and then oh no, I can’t get 7 again so I’m done for the day. Or for example 10, 9, 7.
Instead, figure “Well, either 21 or 28 is close to 25: I’ll go with 28” and then aim for 4 sets of 7. And if you can get more than 7 on the 4th set, all the better. But that is the one you are going all-out on. The others should not be death-efforts to complete.
You could also make those sets start out lighter and work up but that subject has been beaten to death and so I won’t touch it here 
But the point is, you shouldn’t be going into the final set unnecessarily weakened because of having done maximal reps in the leading-in sets.
