I honestly feel your pain, and can understand what you are going through.
I've been placed in the friendzone way too many times to count. I want to say it will get better, but I've learned sometimes it doesn't. Sometimes you continue to stay the friend, but your heart doesn't want to accept that. However, there are somethings you truly need to know.
1. If you actually haven't confessed your feelings, maybe try to do that.
Keeping it bottled up is not good for you, and maybe you wouldn't actually be in the friendzone if you could man up. So take the risk and see if you placed yourself in the friendzone, or if they actually feel like that's where you belong.
2. There was nothing you could do to make them see you any other way.
I used to tell myself if I was skinnier, or if I was prettier he would notice me, but deep down I knew that wasn't true. Once I noticed that my physical appearance was not the cause of him not liking me, I began to focus on personality. Let me tell you, that was a rough pill to swallow because I just figured I was too annoying and that he hated me. To be honest neither of these are the case, not even for you. We put these thoughts into our minds because we think there has to be a reason they don't feel the same way. In reality, it's just they don't feel any chemistry, I mean if it was your personality you probably wouldn't still be friends with the person.
3. Don't think badly of the person, even if that would be easier.
This person hurt you, and I understand that. In fact, I went down the dark rabbit hole that had me always slightly hating one of my best friends, but it wasn't something he could control, the heart wants (or in this case doesn't want) what it wants. So instead of helping our friendship move in a positive direction, I sent it spiraling towards extinction. Luckily I've been able to slightly recover, so I'm telling you it's not too late. I know how easy it is to be mad at them and blame them, but realize they never wanted to hurt you.
4. It's OK that you still have feelings for them.
Don't be ashamed or try to deny it, instead accept it. You won't wake up the next day already over it, in fact it may take months or years before you see yourself begin to move on. But you know what that's OK. Just remember the reason being friends with them can work is because you feel comfortable around them, if you didn't you never would have confessed your feelings. So take it day by day and you'll begin to see it get easier.
I want you to also know one day you may be the one putting someone in the friendzone. So think how you would feel if that person left your life because I've experienced that and I wish they could have seen this article.