People don't think about you as much as you think they do.
People think about you more than you think they do.
Those two are completely contradictory. But both completely true. Some clarification is in order.
People don't think negatively about you as much as you think they do.
That giant pimple that you're so embarrassed about? No one notices.
That stupid thing you just said? Everyone forgot about it.
You can't assume that every flaw you feel you have is being scrutinized by everyone you come in contact with. Because the fact is, that's not happening. That's not reality. Everyone's too busy spending their mental energy on more important things. And the small handful of people that do talk about you behind your back for having a ketchup splotch on your t-shirt are not worth your energy!
I wish I had realized this sooner when I was a teenager! It's so freeing to live your life for yourself the best you can and not spend even one minute worrying about what others think of you.
People think positively about you more than you think they do.
Recently I ran into a girl from my Stake growing up. (In my church, each individual congregation is called a Ward. Several Wards are grouped together to form a Stake. Most activities and church services are done in individual Wards. But often the Stake will join together.)
She's a couple years younger than me, we were from different Wards, and had different groups of friends. I knew her name, but we never really interacted. I assumed she would have no reason to remember me.
But she did remember me! She didn't share any specific memory, just that she loved girls camp and she always knew I would be there. For whatever reason, I was a positive part of her teenage memories.
If you are just living your life, trying to be a nice and good person, people will notice you. People are drawn to good people. Even just a quick smile to someone in the halls can be enough for that person to tell her friends, "Do you know so-and-so? Yeah, she seems nice." If you're trying to be your best self, you can't help but radiate a light.
If you feel alone and without any friends, don't assume you're invisible or being purposefully shunned. It's hard for all of us to go outside our comfort zones and reach out to new friends. So be that person. Be proactive. Assume that others noticed you for the good and would like to get to know you better. Be the one to reach out first.