There is nothing wrong with you. While it may seem that there is, I want you to know that you are perfect in your own way. I know how awful it feels when someone does not invite you. You probably think it's just an oversight, it surely had to have been a mistake. But when people continue to forget about you, it hurts. I want you to know it was probably nothing you did. Sometimes people just can't find it in themselves to be inclusive or even be kind. People might try and make excuses as to why they probably did not want you there but ultimately it is up to you to decide whether or not to believe them.
Rejection on any level hurts, but just think what the other person must be going through as well. It may seem that they are having a fun time, but under all of those smiles has to be a story. Perhaps they see something in you that makes them feel inferior, or makes them feel upset, or not good enough. Maybe the person you were years ago is still the same person they see now.
You, like everyone else in the world, may have changed since middle school, high school, or college. But just know at the end of the day, what others think about you is not what matters. It is who you, in your heart, know you are. That is the only thing that matters. It is hard to not care about what others think but if you are able to look in the mirror and see how beautiful, kind, and amazing you are, that is all that matters. You have won. You should not have to care about people's cruel words or things they have either said or done to you. If at the end of the day you know you are special nothing else should be able to change that.
When you think that you have no friends at all or look back on all of the bad things that have happened to you in the past, try and look back on the good things and people you have know too. Try and think about all of the people who were kind to you and then try and find it in yourself to be kind to others.
Even if that person was a teacher, parent, or old friend, try and understand that at least one person saw how special you were, how special you are. From those saddening times, choose to aspire to be nice and inclusive. Be the person that can make someone smile, and above all find the person that can make you smile. Surround yourself with people that you can count on. Surround yourself with warm, kind people that include you and love you for you, because you deserve to be seen as the amazing person you are. Because you are amazing.