Everyone is smarter than I am

I can just tell by the way people speak to me, both on the internet and in real life. The memes and comments I see on Reddit, the outrageously witty and out-there jokes insufferable loud and silly and obnoxious 8-year-olds come up with on the bus. If I try speaking to pretty good-looking cool trendy hip up-to-date people of either gender my own age, that I wish I could fit in with, within like 5 minutes I am the subject of jokes and too stupid to join in with their banter. If I try consulting others for advice (older adults obviously, anyone younger than 30 will treat me like dirt) even then people tell me philosophy that I never thought of, they justify it with "life experience". All the time people on the internet accuse me of being much younger than I actually am, comments like "You are 13" "OP is the 8-year-old" "This was posted by a child". All the time people in real life think I look 12 years old when I'm actually 18 years old. And they treat me like one.
I feel like as of 2024, after having lived under a sheltered rock and diving into the deep end with a dramatic culture shock, like everyone (unless they're as stupid as me) is pretty much insane, they themselves do not think they are insane neither do my professionals but I deem them insane because I'm stupider than most other people, I can tell by the way they speak and also the clothes they would wear and their music taste and their mentality like everything about them. I've heard kids say:
"I don't know if your shorts are beige. They could be"
"Everyone is stupid including myself"
"You're such a clown"
"You're not as smart as you think you are"
"Only cowards use manners"
And my 9-year-old iphone-addicted stepbrother never takes me seriously and he constantly makes jokes about dying. My 17-year-old brother, that I have had a very close relationship with all my life, is a recluse (that is currently really into the late 2000s emo/scene aesthetic) that is overly judgmental and has extremely unpopular opinions.
There's adults in my life, such as my psychologist, my psychiatrist, members of staff from school/college, that appear to have good opinions of me. Every time I whinge about how dumb I feel they automatically tell me about how "intelligent" I am. I ask why and they explain "You're a deep thinker" "You're very reflective" "You overthink" "You like philosophy", either they are exaggerating that just to make me feel better (they assume it was because I have loads of unread Nietzsche books from my edgy nihilist phase at age 14 or smth) or I am like that but so is like everyone else except they aren't as open about it as I am.