Is normal to lose a best friend?

Here's the thing
I dated this guy for about 3 years. We are currently not together at the moment but we still talk all the time. We are constantly talking about how we wanted to get back together but our breakup was so stupid and ended so bad. I eventually got sick of being upset about it and moved on so I am currently dating someone else. A while ago he reached out to me again before he knew I was dating someone else. Long story short I ended up going back to my best friend telling her everything as always. I still would tell her even now how I still have feelings for him but now she's trying to date him. I feel like she's completely stabbing me in the back. which isn't the first time because in the beginning when we were just talking she tried it then and that was about 5 years ago. I know I probably was dumb to continue my association with either of them but hey i was kind of dumb. Not to mention that we also recently exchanged words which was mostly me saying how much of a terrible friend she is. I know this sounds like a bunch of drama but should i just cut her off or deal with it and stay friends? and was i wrong for going off on her...?