It sounds like you’re quite experienced for your age, so don’t knock yourself on how impressive that is! You’re miles ahead of where I was at when I was 17. I checked out your portfolio and I like your Mind Olympics video, the motion graphics are great.. keep building your skills and pushing yourself to experiment more—you’re on the right track to become a great video editor.
I also checked out your proposal, it looks like in the screenshot that the copy gets cut off after just a couple of sentences, so I couldn’t read the whole thing. That being said, based on what I could read, one area of improvement for you that’d make a big difference in landing a higher percentage of your proposals would be by altering your approach so that you’re speaking in value language.. instead of making the conversation primarily about you, make the way you speak in your proposals all about the value your client will get out of choosing to hire you for the job—and do what you can to provide value up front.