Its a shame that its hurting your feelings and its total understandable. However, she may have just decided she doesn't like you so much anymore now that she's gotten to know you over 6 years. It takes a long time to get to know someone you only see every now and then unlike when people are younger and would spend every weekend and after work together. Back then you could figure out pretty quickly if someone was going to be a close friend. Now its an hour here or there and its with other adults or with the kids. So, you get to know each other in small increments. She may just not like you as much as she hoped that she would. Its not either of your faults. You can't be who you're not and neither can she. Im sure it hurts your kids as well, but if her kids were begging to see your kids Im sure shed give in- so they probably don't care so much as well. Im not trying to be mean about this. I get that its painful. Hey, give her some credit- lying is bad but you can look at it as though she was trying to save your feelings rather than just saying - hey, you're not on my A list anymore. And its OK, being a B list or C list friend is ok, Im quite sure you have a couple of friends like that too.
Maybe you've done this too. Try to think about the actual good friends you have that you don't call often enough. Are there friends you've had for years that you take for granted? Friends that want to be with you but that you don't go out of your way to see or make play dates with just because of time or distance or whatever. How do you think they feel...probably like a B lister even though you tell them they are on your A list.
No one is perfect but we do have the right to choose who to spend our precious time with...and this girl doesn't want to spend hers with you. You said she uses people, why do you want to be friends with someone who uses people and lies? It seems a little clinical to me. Maybe you are attracted to people who push you away. Maybe its a challenge or something. Either way, don't teach any of this to your kids, it isn't healthy. Teach them to let things go, not harp on them. Teach them to love themselves enough to let toxic people go and with good riddance and find healthy relationships.
Join some new groups and make some new friends or reconnect with old ones. You are a good person or none of this would upset you but you need to handle it like a mom.
Good luck.