The first thing you can try is what's called negative ecentuated
When you are fully fatigued, you disengage the assisted function.
Step up and very, very slowly lower yourself down, then very quickly step up and repeat keep going until you are unable to control the downward movement.
So your muscles are only controlling the downward movement.
If your first set is 8, good full rep's
(Then your aim (goal) in say, 12 months is to complete 3 sets of 8)
Now, this is the course...
On the final two sets where you presently fatigue at "5" and "3"
I want you to finish off doing doing as many as possible negative movements
Take note of how many negatives you add to each set, your goal is to add negatives.
Do this for three months.
On the negative, make sure you breath, taking short breaths, sort of like a women breaths when giving birth, sounds funny, but it's a sure way of getting enough oxygen to feed the muscles, you'll need it.