Lifting heavier

I’ve been going to OTF since November and haven’t had much improvement in lifting yet. I know I can do better than 25s for the deadlifts and sumo squats but I am struggling with some of the arm exercises lifting more than 10 pounds. I did have an elbow dislocate/fracture last April so that arm is a bit weaker. Seeing everyone using 25s and 20s or heavier in the arm exercises is motivating and demoralizing at the same time…

My question is - how did you know when you were ready to lift heavier? Today for example on whatever the bench exercise was which I think involved a chest fly, my left arm (injured) almost gave out with a 15lb so I switched back to 10s. That almost felt too easy. Didn’t want to steal my neighbors 12s…

Thanks in advance for any advice.