I can really relate to a lot in your post. I didn't have my first experience with a guy until my early twenties. It was OK but not quite right somehow for me which just made me more confused. Fast forward a bunch of years and I realize that I was with a gay man and what I wanted/needed was to be a woman with a straight man.
When I was younger I had girlfriends, but not that many, and for sure less than what I could or should of had "on paper." Looking back now I see some traits and feelings that I had along the way of otherwise being a fairly "normal" boy. Like playing dress up in dresses and only stopping until I started to feel guilty over how people looked at me.
As far as the tumblr "sissy hypnos" go... I can relate there as well. About four years ago I was surfing for porn and came across one of those and watched it. It was a pretty incredible experience - I felt like inside me a dam broke and all these years of repressed feelings and hidden desires suddenly came rushing out. It caused me to look back on my life and see a lot of "signs" that were not quite obvious at the time. I don't believe that watching that will turn you gay or turn you TG. For my own part it just triggered the release of things I didn't even realize I had been suppressing. If I didn't have anything suppressed, then watching the video would have had little impact.
I would agree with Ms Grace to be careful as porn can be quite toxic and potentially addictive. Personally, I don't like the stuff that is degrading so finding something that is not degrading but in the wheelhouse of what I like can be tricky.
The good news is your on here and can get some really good advice from other people that can relate.
You mention having parents that you believe would be supportive, but you are also deathly afraid of disappointing them. I feel completely the same way. It is amazing what we can and will suppress/endure to avoid letting down those we love. It's interesting we worry most about disappointing Mom, isn't it?
Last few thoughts for you.
You are correct in stating that you are young and have your whole life ahead of you. I am not so young so will give you this caution - what you are feeling is not going to go away... ever. Not matter how thoroughly you purge next time, no matter how many girlfriends you have, no matter how much spare time you spend playing sports. You will never escape from who you are. So...
Take the good advice posted on here previously if you have not done that yet. Get yourself to a good therapist that works with transgender people. Nobody close to you will know unless you decide to tell them - there is no risk. Going to the therapist will help you sort out how to cope with how you feel. The good news is you are young and have a lot of years ahead of you, to transition and live as a woman, or to move forward understanding and accepting all of who you are and with some ways to cope with how you feel without transitioning.
Good luck