what should i do

My ex gf broke up with me 3 weeks ago. we decided to stay friends, and i actually wanted this. I am doing okay some days but when she texts me and we talk a little, i end up asking myself why she wants this. she broke up with me and i honestly feel a little replaced because she made some new friends and spends all her time with them, just when she decided to break up. it seems from my pov that she is either always hanging out sleeping over and never alone either because she doesn't want to be lonely or because she might be developing feelings for one of them (she denies this). now the question is what would you do in this case? i want to keep a friendship and i would never open up to her about how i feel now. but i'm having a hard time moving on and dealing with feeling replaced. and why do you think she wants a friendship with someone she clearly didn't want in her life anymore?