She has A boyfriend but wants me, what to do?

The girl I like has a boyfriend who she claims she loves.
However, She has told me that she likes me and says things like she cant wait untill we are alone together. She is also very jealous of any relationships I have will girls like she got so angry the other day because she thinks im in love with one of my closest friends, who is also my EX.
And stopped speaking to me when I started dating both boys and girl, telling me she cant trust me because I was to fast to move onto ANOTHER GIRl

what do I do?

DO NOTHING... or run away. For way too many reasons to list, this woman does not sound like stable girlfriend material. This sounds like crazy waiting to happen. Steer clear of this one.

Tell her that you don't say anything against the boy she is with and so shouldn't she. And if there is another reason behind her jealous behaviour, she ought to tell you. You can't read her mind. But going out with some guy she "loves" and making account for your relationships is clearly impudent.