There have been many times that I have felt like skipping, but since push-ups can be done really quickly, I just decide to do as many pushups as I can, and then I end up either doing close to my full set, or maybe I just end up doing 70% of what might have had been able to do. .and so that keeps the progress going, even maybe if I ended up doing 30% to 50%, that would probably be better than skipping sessions, yet once I start doing the pushups, I usually cannot stop at 50%, so I usually will want to do more than 70%, and so I will find if I get to 70% and I am continuing to have energy and the pain is not that bad, I just keep going and in the end, I feel good about the session, even though I started out by not wanting to do it.. and even dreading getting started.
I often encounter this and I see it as a challenge, it was hard for me to face it for months as I continue skipping and procrastinating with the thought "i will do it tomorrow " and tomorrow started to feel like some endless loop 
and i began to add weight 
I was only able face it by just giving a little try with the though"just little" and gets motivated anytime I try the little to do more, thus achieving the daily sets
, from little to achieving more than expected is a good achievement for everyday fitness and there's no fitness without pain IMO And yes, no fitness with no pain.
No pain?
No gain