Discouraged

So, I hate coming on here and venting but I just need to say it...so I'm sorry, I don't mean to offend anybody.
So, I had my 18th birthday party last sunday and it was crap. I wasn't even the reason for the party by the end, half the time I was inside because I knew I would burst into tears or scream at somebody if I stayed outside. I hated it, because I felt so selfish, I should just be thankful for what I have. When I was opening presents everyone was talking and nobody even cared what I was doing except for like one person who was sitting beside me, like do you know how worthless you feel when that happens. I just don't know why it is bothering me so much.
This whole week hasn't been much better, I feel okay and then all of a sudden I am blinking back tears, I feel very alone right now, even though I know I'm not. I feel like a failure and it sucks. Like take today for instance, I was at skating practicing with my dance partner and he was explaining something to me and all of a sudden I felt like crying, he wasn't being mean or anything, I don't know what is wrong with me. I feel like just crawling in a hole and staying there forever.
I feel like people expect either extreme greatness or they expect me to fail. I feel torn and confused. I just don't know what to do.
Anyways, I'm sorry for wasting anybodys time who read this, I hope your day is going well.