Diet and exercise are big, particularly as you get older. Your body can handle a little bit of crap, but not a lot.
Testosterone supplements OTC are one thing. Testosterone therapy is another. If you go down that route it lasts the rest of your life.
Don't be afraid to get a second opinion if the doc recommends injecting T. I'm sure some docs are well rewarded by big pharma for creating a long term buyer of their products.