Hello!
I am sorry that you feel that way. It is not very easy to solve situation like this, especially if you feel you have no options. But there is plenty.
I know being alone feels like myself, and the only advice which can help - or, at least the only one we can give you online without knowing the situation properly - is to meet new people.
It can feel pretty isolating when you think that all people have friends already so you can not meet none else. But, the true is that there is always somebody in similar situation, or other people willing to include you. I found new friends myself by coincidence, as I am from the Czech Republic and I found other Czech students in a city I study now.
If you feel like these friendships you have already are not worth it, maybe it would be better to accept it and try to find new friends. You can do it via facebook (e. q. postgraduate students have groups and it should be relatively easy to invite them to a pub etc). It is a cliche, but similar hobbies are great for making new friends.
The other option, but the less probable, is to stop expect that they will do the first step. It really depends on how the relationships look like and how the people behave towards you. But, sometimes you may be (I am just quessing) less active. But, when you want to be a part of already formed group, you have to try. It means e. q. asking them what they are gonna do (maybe they are going to a party and they will invite you), try to be friendly (e. q. buy a bag of chocolates and share them during lecture - if it is allowed to eat there, ...). It definetely should not be like trying to much - but it is sometimes good to show that you would like to go to the party, that you are funny, etc.
I hope you will solve your problem quickly and won´t be alone any more!