Some days I feel less energetic than others. I've been back to work for quite awhile now and generally doing alright, but I notice my mind wandering at times. That being said, getting back to routine and feeling normal was something that I really wanted and needed.
It always makes me feel better after I get some exercise, whether that means I go out for a paddle, a run, hit the gym for a boot camp session or just do some bodyweight exercises or pilates at home.
Make sure you feed your body properly as well. Keep that protein intake up. Eat the right amount of calories and make sure you're generally hitting the correct range of macronutrients (e.g. 20-30% of your calories from protein sources, 20-30% fat and 40-60% carbohydrates). Don't miss the fruits and veggies! It can be a tough thing to do when you feel like you don't have energy to cook properly. You don't have to be perfect about it. Have a cheat day here and there, but keep the majority of your days clean and your body will feel it. Also make sure you're drinking enough water.
Proper diet and exercise are the two things that are directly under your control. Well, those and making sure you get the right amount of sleep. Not too little and not too much either. Easier said than done when you're stressed. The rest of the stuff that might be making you tired isn't so easy to control and therefore isn't worth dwelling upon. Yes, cancer happened. You had the surgery and the radiation and that took a toll on your body (and the rads may still be having an effect). I know nothing about menopause (covers ears and says "la la la la la la") other than it made my sisters and some of the ladies I work with a little bit cranky, and unusually hot at times. Sounds like you're getting checked out at the doctor's office(s), so presumably they've looked into all of the possibilities and addressed them.
I wish you well and hope you find that energy soon! Good luck with the return to work. They started me out on half days at first and I found that helpful, although I really just wanted to get back to normal the whole time. Haha thinking back now, the lazy part of me kinda misses having those half days again.