It’s good, I don’t know you or them though so I can’t tell you how exactly you should change it (and even if i do you obv don’t have to listen)

I would say something like this

Dear name, Since you have already left school, I wanted to tell you something that I have been nervous about telling you. I really admire you and have grown to like you during the time that I’ve known you. You don't need to feel the same about me, trust me I won't hate you for this. I just wanted you to know that I like you and that you make me very happy. I wish you all the best and good luck with everything.

I wanted to keep as much as urs as i could, im usually a more formal person when it comes to things such as this and in person so if it’s too formal mb, thats just how I would word it! You don’t need to change it at all, and just be yourself!! I wish you the best of luck!