I'm trying to gain muscles, and it is SO HARD to work out

I start out nicely, but at the end it is the most horrible thing when you don't have energy to do a machine

How do you overcome this lack of energy?
I know that if I do lighter weights, then I won't gain muscles, only stamina (right?)
Pushing limits is the worst

I feel so spent and weak
I try to do around 70 pounds on machines