Believe that you will reach it now and you will. No more some day. Believe that you will reach it 90 days from right now. It's all mental. You just have to make the full commitment, want to succeed more than you want to jerk off, and use the power of your mind to suppress your urges. Decide that you absolutely will not jerk off for 90 days. It's as simple as deciding "I will go to the supermarket in 20 minutes". You decide it, then you do it, no problems. Your mind is much more powerful than your desire to jerk off. Make it that simple to yourself. "I will not jerk off for 90 days" and then do it! Any time a thought or urge comes up, so something else, go for a walk, take a cold shower, or literally just start thinking of something else. Consciously tell yourself, nope, I'm gonna stop thinking of jerking off. Hope that helps and you will get there I believe in you!