Amen to your last line: "I feel 18 again and realize I I thought it was the end of my life, but it turned out to be the beginning." The only difference for me is that I was miserable at "chronological" 18; so I've been 18 at heart only since January. I'll turn 62 in June. It's "all good."
I was 14/15 when I first identified as a lesbian. At that time it felt liberating but the older I got the more horrible I felt at the fact that I was 'actually' gay. When I was 17-18 I started identifying myself as bisexual in the hope that I wasn't 'truly' gay. But I'm 19 now and I know that I will never be able to change the fact that I like girls, but I'm still in the process of accepting this as a true part of myself and my life. It's a long journey to say the least but knowing that your sexuality will never change is the first step to acceptance, in my opinion.
Yes, it is sad and hard on us when that happens. It would be nice if we could go back in time and be the one to stop it earlier ourselves so that we didn't have to endure their fickleness and selfishness. Alas it happened though. Just like other life stuff happened which we wish that we also had better foresight of prior to it.