Endurance is being able to do an activity longer, so to increase endurance you need to do the activity longer. You're talking about endurance on the rather low end of the endurance scale. In other words, not on the doing an activity for several hours continuously (such as cycling or marathon running) where the word endurance is most often used. What you're looking for is sometimes called power endurance.
Also there's always a trade off with higher weight and the reps you can do. For example your 1 rep max is your max because you can only do it one time. You can do a lower weight more times. So it's possible your endurance isn't the problem and it's just that your expectations with what weight you can do for your whole set is wrong. But it is also possible with the same 1RM to be able to do more reps at 80 or 90% or whatever of your 1RM. Again to train to do that means you need to do more reps.
Some options to increase endurance:
- Lower weight and more reps. If you can't complete your sets at full reps try dropping by 2-5kg until you can. Then progress adding weight from there as you can but still completing your sets.
- Add additional sets with lower weight and higher reps. For example try putting 70-80% of the weight you normally workout at and try aiming for maximum reps.
- Add 1-2 negatives at the end of your last set or alternately at the end of each set. Have your partner give the minimum assistance to keep the weight moving as you tire out. These can be extremely challenging and if done at the end of each set will tire you quickly meaning you will not be able to complete as many sets.
You may need to vary between these options to increase your endurance.
As with increasing any exercise program, monitor carefully for signs of overtraining where your body is not recovering properly.