How to use my energy in long sessions.

Hi. Newly into the Intermediate 7 title guy here.

I'm recently been into playing for longer sessions (2 hrs), but I've found myself particularly exhausted and no longer in my full potential after roughly the first 30 mins.

What usually happens is that after I pass a few S16s, or D15-16, I find myself without much stamina to keep playing these difficulties consistently. Usually I end up lowering the overall difficulty to S14s or D13s or lower.

What should I do to keep some of my energy overall in the entire 2 hrs of session?