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