What should i do?! help me!!!?

I have been in a relationship with this girl for 5 months, we have had our differences about thing, we've been in fights about things...we worked it out, but there is still something that bothers me...she has been in a relation ship 6 times in a row, and in love 3 times in a row, she has called me her fianc�e a lot of times. but earlier i found out that she has also called the 2 guys right before me her fianc�es too....all of hr previous ones ended out of no where for some reason and im not quite sure why, "they just ended" ...thats what she told me, but i found out from her last BF and my ex friend that she is naturally promiscuous, he dated her longer then any other guy (1 year) and he told me that she lie's alot, she lied to me a bunch of times....i dont know if i should break up with this girl knowing that we might end like her other finacees or stay wiht her and risk getting hurt, if not those two then what should i do? (ps: me and her talked about her lying a bunch of times and she still lied after, it was about our communication, i asked her to stop blogging like crazy so we can have more involved conversations, and she went right on ahead and blogged her off, the thing that bothers me is that she said she would stop, if she said she wouldnt i would have agreed, but she lied, thats what bothers me....oh and i heard she gives away her kisses as if they were candy or a gift to a friend, she called her self bi ual for 4 years, and suddenly she says shes straight, should i believe that? i need help and good help with this...and i also heard that she t a guy and went to his house an slept with him no questions asked (her ex BF called her a smutt anto her face and didnt take it back, not once) should i believe him, is she not worth trusting, and he also said that shes not worth it and that im wasting my love on her, i dont wanna believe him but his letter was so convincing, and it was writen in such a short time...he seemed to know her more then she knew herself, should i take my ex-friends advice and stop dating her, or should i keep dating her and risk my heart and love? i need help, thank you for reaing and sorry if there were typoz im kind of in a hurry...if you need more info ask me, ill tell you anything, im at the end of my rope right now, i need help, what should i do?!