Sora doesn't really think in terms of fair! This is what he wants. And geez, Sora's pulled fast ones more than once in his day, but he knows how promises work. No technicalities! Not for him, at least.

"This is something that could save someone's life if I'm careful about how I use it. After I fix what went wrong this time." And fixing what went wrong this time is sort of a goal he'd had outside of this! It just so happens that fixing it will fix a whole host of other problems as well. "I can't make a promise that I won't use it again if it comes down to hurting myself a little in a way I can fix and letting someone else get hurt. That's... just not an option to me. It's not."

When it comes down to it, that form can do so much more than Sora can - and moreover, it'll do it without hesitating or getting caught up in emotion. Of course, it's still not great for his self-esteem, but Sora is only thinking in terms of two years of trying to save everyone, not two years of trying to fix himself. (The idea that he could try to have both is just not occurring to him at the moment.)