"Any sport is 90% mental, 10% physical when it comes to training"
If you ever go into a basketball game feeling like you're gonna lose, chances are you'll lose.
The same thing with fs, if you feel like you suck dick that day, you'll suck dick...
You need a positive attitude, my old trainer said if I didn't feel good about myself that day, don't bother going in. It'd be a wasted session.
There's a difference between physically tired and mentally tired, decide which you are, find a way to fix it. Eg: rest/cheer Yourself up.
mind > body