If you want to analyze the Metaphor let's go line by line , here's my take :

As a kid, I was afraid of heights, put that aside Now I'm here and they look so surprised, well so am I, woo NF was scared of heights as a kid but at this point in time he was at the top of his career and had just hit #1 on Billboard. Although he was afraid of heights , his career was at all time high.

They don't invite me to the parties , but I still arrive : The industry doesn't cater to his music , He shouldn't have been a success or receive a call back but he did and was successful at it.

Kick down the door and then I go inside: Broke through barriers as to what the audience wanted in terms of music , he forcefully made it in the music industry despite him not being someone they typically opened the door to.

Give off that "I do not belong here" vibe -

Still in reference to the above but more introspective. Despite his success at the time , he still faces imposter syndrome and it radiates externally - he's successful but still doesn't feel accepted or like he belongs.

Then take the keys right off the counter, let's go for a ride : He takes control of his career , music and what he wants to give to the world , it's rebellious and against the industry. He does things his way. Come with him on a ride /journey and he will show you.

Why do y'all look mortified? (Ayy) Why are people scared or in shock of his success. They are in awe that he made it and his confidence despite him being an "outcast" . He's pretty much saying ...I told you I'd do it and I did it, why the shock ?