You do need to lift heavy to get stronger, but if strength gains are your priority avoid going to failure on every set of every exercise in every workout. I say this because this is what had me plateau at the exact same weight as you for months. I believed I had to push more to get stronger (I got bigger, but not stronger), but constantly grinding out reps burns out your nervous system, reinforces bad form and you gain little to no strength. Lift heavy but leave a rep in the tank at the end of each set so that your form didn't break down. This might mean you need to drop the weight a bit and build up again, use less reps on your current weight, or bump it up to 70kg for a single rep. Actually I found for me if I bump up the weight, do a single, then drop back to my actual working weight it feels lighter and I can get more reps. If you want to, only train to failure on your last set and go for as many reps as possible to measure progress. If you get more reps than last time, you got stronger.