At your age and level of training, your joints are fine. The work they get from the 15s and 10s is plenty. Don't over-complicate things. Just lift and eat, it's so straightforward and it works.
Are these weights you're using in kgs or lbs?
God help me!!! Kilogramms
If you aren't able to do negatives, then keep trying to progress the weight on the fives for as long as you're able. Then take one day each to retest your 15RM, 10RM, & 5RM weights, 9-14 day SD and start a new cycle based on your new RM's.
thx man. So you mean staying at this weight till I get stronger and then raise it again? Because that idea will difer from the weeks before were you just increase the weight independent of "getting stronger".