There's a huge lot of positives starting at 15; its about as good as it gets. I've seen so many people post questions on Internet forums asking it is too late for me at 60, 50, ... 40, 30, 20, 18? No one asks that at 15.
There's a lot of people here, like me, who started much later in life and its a very hard way to go. Like many I've a lot of regret about not starting earlier. Also as you get older you accumulate a lot of ties to your old life and its gets harder and harder to change. I'd imagine your physical results will be pretty well perfect.
I'd think you'll live a lot longer on HRT than you would without. Being trans and fighting it is an horrible way to live, and very unhealthy. I've seen my blood test results change with my stress levels. If I'd kept at it I'd be dead of an avoidable medical condition sometime in the not too distant future. That's not even counting that living like that is not really living.
Its very normal to question yourself, sensible even. At the end of the day there's no way of being sure about anything, but you've have to make decisions anyway. There's no certainty in life, just do the best you can.
I may be wrong, but I think the usual HRT at 15 is puberty blockers, so I don't think you're really making any serious commitment yet. Just delaying the real decision time to a bit later.