Choosing Weight 

A good weight should challenge you while allowing you to maintain proper technique.

Ask yourself - Could I have done 1-2 more reps?

If yes, you're probably using an appropriate training weight.

If you could have done more reps, it's likely too light a weight and you’d be better off choosing something heavier.

If your form breaks down after the first few reps, you can put the heavier weight down and do the rest of the set with lighter weight.

Don't be afraid to challenge yourself. In fact, you need to challenge yourself in order to see change. Getting stronger requires your muscles to work.