An irritating friend

[MegaFullbuster]

I have a friend, one of my best friends who I have a great deal of fun with. We laugh about the same stuff and just have the time of our life together. We often bicker like an old married couple and such which makes people think we are dating.

But there is one thing that I cannot stand about her and don't know how to deal with.
Whenever there is something she wants to do that I don't want to do she gets upset and bitchy towards me, so sometimes I'll just give in and do it so she won't be mad but I try not to encourage her. And then there are times where I agree to do something but then later have a problem with it or just have changed my mind and no longer what to do it and then she gets really bitchy and pissed off. I tell her that I can't help it I just don't wanna do it or I just don't have any interest in it or I am doing something else but it doesn't stop her from getting pissy. And whenever I bring it up she just gets pissy and is like okay whatever and completely shrugs me off.

I don't really know what to do and I don't have much experience in dealing with peoples issues and such so I am hoping someone reading this does.