I so remember this...your body is doing a lot of work and burning a lot of energy with the baby...and yes, it is definitely more difficult when you are over 35. I was exhausted my entire pregnancy.
The best thing to do is listen to your body. Get more sleep, maintain some activity level (e.g., 30 mins of walking), and maintain a healthy diet. They all go together in providing the energy your body needs for the pregnancy. If you don't maintain some activity level, you will actually be more tired. If you don't eat a balanced diet with lots of protein and vegetables, you will be more tired. And, rest goes without saying.
3 more months to go...take this time to take care of yourself. Good luck!