Idk what to do

So I’ve been talking to this guy for 9 months and I’m really starting to fall for him like hard but the thing is he’s my best friends ex. They dated 2 1/2 maybe 3 years ago but I still feel like it’s fucked up. I don’t have the heart to tell her I really don’t want her to be mad at me or to not wanna be friends anymore. I really don’t know what to do, before I started talking to him I use to cry 2-3 times a week from past issues and now I barely cry about them, he makes me feel so good about myself and I know I can talk to him about anything, he just makes my day so much better. But I don’t know what to do. Please help!!