I have had a similar thing happen and all I can say is that while they are in another relationship it's like they are living a fantasy - La La Land as a friend of mine put it. Rebound relationships can work but it doesn't mean that they are actually really happy.
Give yourself some space - I have now I am going through 'no contact' and it really feels like I am in control now
I think she needs time to miss you and it will give you a chance to decide what you really need in your life too.