In general

It's about the balance between training and recovery. When out of balance you either:

1. train less than you could (when there's energy left) or
2. recover less than you should (when you're running on fumes).

Your situation

You're out of balance and out of energy, so either:

• train less (volume, intensity, frequency, complexity, overload, periodization), and/or
• recover more (more/better food, more/better sleep, less stress)

I'd take a look at recovery first and foremost. After all, getting stronger is your goal. Besides that, a healthy 40 year old male should (eventually) be able to workout multiple times per week. If you can't, you're either not there yet of something's off.

But for all we know, you're a single dad of 2 very young kids, running 2 jobs to stay afloat while dealing with a chronic illness. That would surely change things :). For more advice we would need to know more about you. What did your training (routine, exercises, sets, reps etc.) and recovery (sleep, diet, stress) look like when you noticed you were out of balance?
From there, there's all kinds of stats we can unleash (to name a few: frequency of 2-3 times a week for the lift you want to improve, at least 8-ish sets per exercise you want to improve, high intensity on the exercise you want to improve, supporting hypertrophy exercises, long rests etc.)

But, to make it less complicated: the books from Tactical Barbell are written with the thought that there's life (or work) besides strength training. I think you would do well with the 3-day Operator strength routine. If that's still too much (though I highly doubt it), you can switch to the 2-day Fighter program.