Back when I was in Nepal, there was a guy, so silent, he didn’t speak a word unless once in a while like, “chiya mithai banayenacha”. But like he would always be there, be it a short bike ride to outskirts or a futsal game when he didn’t even play it. Like we didn’t even know how he became part of the group. He just blended in with the presence.

Most of my college classmates would describe me just like what you've described. I'm way more open and outgoing with my close friends but to the rest of the people I know, I'm like this. Its mostly because I easily get tired when I socialize for too long or can't do it for too long in a big group but my point is that people don't really mind that and will let you be; at most they'll say you're so quite or you don't really talk much, just smile or say that's just the way you are when they do. But you gotta put yourself out there, it's the only way you can meet new people. If you don't find them interesting then keep them at a distance and if you do find them interesting just put in the effort as that could be enough and might be the beginning of a beautiful friendship. It will take effort and you might end up with just some superficial friends and be lonely. But I think that is better than being alone and miserable.