How to work out better and get better results?

I am a 15 year old male, and am in a decent shape. I can do up to 70-90 good form pushups, over 100(good form) squats, and can use up to 25-30 pound weights when called for. I started working out seriously about 5 months ago, and my biggest problem has always been motivation. Now I don't mean motivation in doing it, but motivation in doing it until I can't do it anymore. This has been frustrating me ever since I noticed that. When doing pushups, once my arms get tired, it's hard, and sometimes impossible for me to continue. Pullups are ever worse, I have real difficulty pushing myself on it. What I'm asking of the nice people here on the site is to maybe tell me of some ways to condition myself to exert myself more, or maybe give me a few helpful tips. All comments are appreciated, thank you!