How can I talk with someone about what I feel?

Well, I'm hear to listen if you want someone to talk to. I'd like to help out. I have the same experience you do with helping someone else and then not having anyone to help me out. I'd be glad to help and listen if you ever need someone. :)

My interpretation is that you expect your friends to extend to you the same listening skills and support that you give to them. While this might seem fair, it does not reflect reality and therefore sets you up for disappointment. Some people are good listeners, and others are not. Whether or not somebody is a good listener does not correlate to their need for a good friend who will listen. Maybe try seeking out people who have proved to be supportive listeners of others?

I think you could anticipate a bit your part of the sharing. You listen to them a bit, then try talking about yourself a bit, even if it's small stuff that worries you. It will be easier to see if they are up to do their share of listening when you will need to talk about the heavier stuff.
That said, some of the people you will talk to will be just temporary presences in your life, there's no way around it. Try to treat all of the new people as potential good friends, cultivate many of them, so you don't depend only on one person, setting yourself up for failure by creating too much pressure.

True, but still I wouldn't be comfortable, because I know they would leave me soon without thinking about how sad I would be if that happened...