Do you keep a record of your lifts? Just keep track and constantly improve it each session.
Say you start at 20kg dumbells and do 3 sets of 5, 3, 2. Record that, and next week do better, even if its only one extra rep. Say, 5, 3, 3. If you don't get the extra rep you need, just add a 4th set so that your total number of reps is greater than your previous session. Once you hit 3 sets of 8-10 (whatever your goal is) up the weight to 22.5kg dumbells and repeat.
Just be aware that you will have shit sessions where you won't beat your previous best due to fatigue, poor diet, stress, illness, etc. On those days do your best but don't stress out about it.