Ummm...yeah...you do put too many stipulations on others. You say you can't stand hearing about other people's kids [I'm paraphrasing] but you need to consider this --- as your currently childless friends age, many will get married and become parents. Would you dump a long time friend because she became a parent?
Not all friends follow the exact same path in life. Some will marry and become parents while others will remain childless and not marry. Some will be stay at home moms while others will pursue a career outside the home.
True friends accept that life paths will diverge but will still be there for each other. For example, my Godmother was my mother's best friend. They met in high school and became lifelong friends. My mother married and stayed home with us kids. My Godmother had a glamorous career, married later in life and never had children. While it was clear that my Godmother wasn't into raising kids, she still treated us kids well and we enjoyed her company.
A big part of friendship involves listening to your friends talk about what is going on in their lives. Even your childless friends aren't going to have the exact same interests that you have. When a friend talks to you about something she is interested in but you find boring, do you listen to her or do you tell her you don't want to hear it?
Try this as an exercise --- Let's say that you have a hobby that you're really passionate about. You are excited about what you are doing and want to tell your friend all about it but instead of sharing your enthusiasm, your friend tells you bluntly that she's not in the least bit interested. How would you feel? Would you still want to be her friend?
You ask if anyone has been disappointed in people --- At one time or other as we go through life, we all will come across someone who disappoints us. For example, people who have disappointed me were those I tried to befriend but didn't want to invest the time in maintaining the friendship.
You say you are a people person and love learning about someone new and where they came from. Yet you also say you don't want to hear about anyone's kids. Do you see the contradiction in this?
You wonder how you can have a better attitude --- from what you've written, it appears that you suffer from depression. I would suggest looking into getting help to deal with your depression. Start by talking to your doctor.
Finally, just be open to meeting and talking to people. Approach it with an open mind. Don't think to yourself things like "this person has kids" or "this person is older than I am" or "this person is too busy to party". Just take the time to get acquainted. You just may find that someone you thought you had nothing in common with will turn out to be a good friend.
Good luck and get going!