going to failure and pushing your limits builds strength/stamina. Not doing 80 or 90% effort bullshit. if you feel you were overtraining, just have to reduce sets number
it does build stamina, and i was able to do set after set to failure. but the amount of weight i could lift would never increase. your right though, all i had to do was reduce my number of sets. but if i do that, i end up just doing one, two, maximum 3 sets per workout. and i have found that i get much better results by staying away from failure entirely, and doing lots of sets. i still use heavy weights, just stop 1 or 2 shy of failure. that way i can do more volume. and i actually gain strength. i dont gain much, if any, strength going to failure. but like i said before, i think its because i am an experienced weightlifter now, and my body is not going to grow if i am doing set after to set of failure.