I think my friend are just trying to use me.
Why do you think this way?
Because they ditch me as soon as they get what they want.
It is wrong to use friends to get what you want. You should be honest with them in what you want and try to not manipulate people.
It's my free time and if they want to do stuff with me, they should just do it. They can't expect what they want out of me.
You can always tell them you have stuff  to do and maybe next time. There's nothing wrong with having alone time sometimes.