I agree with the comment about getting enough healthy food, protein and enough sleep.
Maybe there are other forms of exercise that are gentler than the gym. We all have different bodies that have different needs. Plus this may change over time anyway depending on circumstances etc.
Try going to a health shop.
Check for allergies and food intolerances - despite what some claim, there aren't really any valid tests for this. The best test is to cut out only one type of food for about 2 weeks, then introduce it to see if it makes any difference (keep a diary of symptoms and food). Then repeat for another type of food on a separate fortnight. The most common are wheat, gluten (which may or may not mean a GP test for Coeliacs Disease), diary.
Sensible things include cutting down sugar as much as possible if it makes you bad, caffeine (including coffee, tea, cola and chocolate), alcohol, chemicals in food (and maybe products like house hold as you can buy natural ones).
The other thing is to ask your GP for blood tests to eliminate possible causes e.g. anaemia, thyroid, adrenal, etc. Ask if you can always have a copy of your medical letters and blood test results. The GP may refer you to a hospital specialist depending upon your symptoms and results. This is nothing to worry about as it is just a precaution but should be followed up and taken seriously.
You can also go anonymously to a sexual health clinic with or without your GP knowing, and have some blood tests and swabs done there. Nothing to be embarrased about. It's just educating yourself and looking after your body. Some things are invisible so just eliminate the possibility.
You can also take vitamins and minerals from a healthshop (in moderation). E.g. magnesium is just one common cause of tiredness (don't have too much or too little).
It all depends on your body.