There were so many times growing up that I would sit alone at recess or spend my weekends at home reading instead of going out with friends. There were a lot of times when I would sit at lunch with my friends and they would talk about what they all did together over the weekend and I'd wonder why I hadn't been invited. At one point in 10th grade, I deleted every single social media account I owned because I couldn't stand seeing everyone's posts hanging out with their friends.
Something that has taken me 19 years to realize, something that I'm still trying to accept, is that it's not intentional. Sometimes, it feels like it is and that sucks big time. But, over time, I've realized that nobody specifically says, "Hey, let's all hang out and let's not invite Sam." Nobody makes plans and specifically excludes me. That's just not how it works. It's tough to not be invited to things, but it's life. You'll get through it, I promise.
Life isn't always sunshine and rainbows, and sometimes you just have to learn to be alone. Personally, being alone is something I'm not quite good at. If it was up to me, I would never be alone since I love just being around people. However, there are times when nobody can hang out, and that doesn't necessarily have to be a bad thing. Find something that you can enjoy doing by yourself, whether it be painting, reading, singing, etc. Join some organizations - you'll make more friends through that.
It's hard to not feel put down when you seem to never be invited to things, but you have to push through it. I have spent weekends here at college sitting alone in my room and just pitying myself because nobody wanted to hang out with me. I've literally just sat here in this bed thinking, "Why don't people like me?" and letting that bring my confidence down farther. I'd validate that by thinking, "If they actually liked me, they'd ask me to hang out, right?" Then I remember that I don't ask them to hang out either. It's always a two-way street.
If I ask somebody to hang out and they aren't busy, they usually will. Sometimes instead of pitying yourself, you need to be the one to initiate something. And if you initiate and it doesn't work out, try again. Another thing to remember is that there's always going to be someone that dislikes you, but for every person that dislikes you, there's going to be two people that love you. You will find friends that will make you feel welcome, I promise.