If weights are "heavy" for you, you won't be able to do many reps with them, simple as that.
That`s was most people think. Heavy weight = low reps, light weights = high reps.....
But i know some people you can bench with 85 % of there max. for 8-10 reps, and others who only can do this 5-6 times (and the rep speed is equal). "Mind Power & goalsetting" plays a big role..." "i will, i want, i can.."
But there`s also other options to do high reps with high weights, a Training system called Pitt,
you use a weight that you can normally lift or press for around 8 reps, "that`s a fairly heavy weight" (the stimulus/resistance). Now you start the "pitt-Set", doing 1 controlled rep, replace the weight for only 3-5 seconds, doing rep Nr. 2, replace for another 3-5 seconds, and so on, after rep 8-10, your rest between reps
will be around 8-10 seconds, after rep 15, maybe 12 seconds, and between the last 2-3 reps up to 15 seconds. Normally you can do in that fashion 15-20 reps with a heavy weight, and a big number of those reps "where close reps" near to faillure (but not reached faillure). With such a tactic your muscle has the chance to replace "metabolic waste" (lactic acid, ammoniak, ...) by a certain degree, and can restore a bit
of ATP. If your muscle is from the beginning up to the end of the set under tension that`s not possible, and faillure mostly occurrs because you can`t flex the muscle because he is full of waste products and can work
efficient.