As you are not accustomed to lifting heavy weights you may severely underestimate your capability. For example, an 80 KG person who can walk up stairs 2 at a time has most of the leg strength needed to perform a squat at 80 kg load, without any additional training. Consequently, I suggest you proceed with the same increments you have been using, but at the next workout video your sets. This will enable you to objectively assess any developing form degradation much more accurately than you can during the set. Having been through linear progression a few times due to interruptions in strength training I can assure you that the sensation today's set took all you had is a very poor predictor of impending failure.