Does my withdrawal from watching forbidden things after I had opened them count as a relapse from my repentance, knowing that I did not practice masturbation this time? And does my repentance become void?

What happened to you does not nullify your repentance. Rather, we hope you will be rewarded for striving against yourself and closing forbidden pages. Continue with your repentance, block every means that could lead you back to what is forbidden, choose righteous companions, and frequently attend knowledge lessons, ذكر (remembrance of Allah) circles, and Quran recitation. Strive in supplication for steadfastness.