If you have a crush on someone what's keeping you from confessing?

The thought of him rejecting me is scary to me and i feel that he likes some other girl whos way better than i am.

He's a grown adult and i'm 13... 

He's taken and often talks about his gf, not only that but he probably has no idea who I am even though we went to the same class.

Alright, the short list

-rejection sensitivity is a bitch

-I might have abandonment issues and the thought that he might not even want to be around me after learning my feelings for him makes me feel like dying

-I don’t know how to phrase myself

-I feel like our current relationship is in a stalemate bc I’m intimidated and don’t how to approach him without making my personality something completely different so I seem like I’m perfect for him (see abandonment issues)

-I am pretty sure he doesn’t like me back

Because I know damn well that the person does not see me the same way and I will get rejected.  And confessing and then getting rejected is humiliating.

Its a mixture of the fact that I know I wouldn't be good for them and the fact they are currently in a relationship.

edit: My computer was lagging I didn't mean to comment this three times. ;_;)

It's a combination of me not being 18 yet plus a fear of being rejected (for literally any reason).

I like him as a friend bc he's really cool but also I worry he'll think of me differently

I think it's mostly the fear of being rejected. Like I "know" his friends (more like I know how they act cuz I don't mostly talk to them) so I'm afraid he has similar style how he looks at girls etc. Also I'm mostly insecure so it's hard for me to let go of the thoughts he would see me the way I see myself or he will make fun of me. Things like that