what is wrong with her?

Well, first off, a best girlfriend should not be a factor about moving 30 miles away from where you live right now. You are with your BOYFRIEND, not her, and he is supposed to be the most important individual in your life. If he's not, you're with the wrong guy. I don't understand your rationale.

You have been friends 18 years. Sometimes friendships fade and we out grow them. Many people walk into our lives and right back out. Other times, we have friends of years, who suddenly move on. The important people in our lives are the ones who stay.

This friend didn't stay.

You didn't do anything wrong... she's seemed to have moved on. You are reading too much into this and you seem to be thinking YOU have done something wrong.

you haven't

You think of her as your bestfriend but she thinks of you as someone who is convenient to her. She's jealous and manipulative. Some "friends" are like that and they are totally unaware that their behavior is not good, and in fact is unfriendly. I suggest you stop trying so hard to stay in contact with her. It's horrible when friendship go rotten, but you do have to let them go and move on.

What is wrong with her?
every time I plan to do something with my bestfriend she makes up a excuse either that shes babysitting or working but then shel forget shes said that to me and send snapchats saying shes with someone else or shes bored and wants some thing to do? but she always lets me down at the last minute..shes even got me to take time off work for us to go on holiday almost getting me to pay the deposit to turn out she lied about getting her passport? she has been my bestfriend for so long. we was born together and brought up as neighbours, our friendship has lasted 18 years so far and lately shes been doing really petty things like sharing photos of things about bestfriends and tagging these girls that shes known a couple of weeks saying there so friendly their like her family? my mum reckons its the fact I have moved out of my mums house and got a place with my boyfriend and she doesn't like it? when she split with her boyfriend who was horrible to her I was there for her everyday and now when im not feeling the happiest shes no where to be seen? if I post a instagram of me and someone she will message me saying 'whos that is she your new bestfriend then' I thought she was joking at first... im thinking of moving away about 30 miles from where we live now with my partner, and at the moment nothing is stopping me. she just isn't there for me now. why has she changed? what did I do wrong? btw I am 19 next month and she is 18