Dreams are a way that we process emotions and information. I understand that it can feel annoying that she is the first thing you think about after you wake up but just see it as a way of your body processing things that are bothering you (either consciously or subconsciously).
I think sometimes dreams can be very insightful/telling about how we feel/or what we know subconsciously but ignore. As an example I want to share a dream-experience that was quite important to me. I once dreamt that I was shopping with my ex at Ikea because in the dream we would move in together. However, something about her scared me. She was ugly with baggy eyes and wrinkles. In the shopping basket I found a questionnaire that had the following questions:
- Is your current partner someone you can trust?
- Is your current partner a team player?
- Is your partner someone that communicates well?
Etc.
I had to answer 'No' to all the questions. In the meantime she kept trying to look over my shoulder to see what I was reading. I became very annoyed up to the point that I yelled that she needed to back off, go away, and leave me the f**ck alone. I just kept screaming at the top of my lungs: "Leave me alone! Leave me alone! Leave me alone!" Then I woke up.
Not all dreams are this clear of course. But I think they can contain pretty interesting "messages" if you pay close attention. Feeling 'replaced' by someone else is incredibly painful so I don't find it strange that you see them together. [on a sidenote; I dream very vividly/also experience lucid dreams. As a creative person I find this to be a blessing but it can also be a curse: I often suffer from insomnia/sleep paralysis].
Probably the dreams will become less frequent or change topic when you have healed more and are longer apart

Completely cutting contact is a big step and can be a reason that you are now experiencing all these dreams; you have cut cords with someone that meant a lot to you and your subconscious is responding.
Very good of you to also inform your friends. I didn't do that in the beginning and received some pretty painful comments via others.
Stay strong. You got this.