How does an avoidant person "nix the fantasy"?

I find I myself depend largely on fantasy as well. I often fantasize about how my life could have turned out were I to have made different decisions with hind sight. This is prolly common among anxious people as we tend to ruminate a lot. I think it also allows me to imagine I'm that person that was able to behave as in the fantasy because I mentally take myself through the experience. It's a coping mechanism and prolly not the best. It probably distracts from real growth. I also have non rumitory fantasies that basically center around what I could be. It's so much simpler than devoting your energies to how you could make your real situation better. In fantasy people behave as you expect and any challenges you face are ones you think you can handle and are prepared for. Coping with life as it happens is so much harder.
I'm sure fantasy is useful but I imagine insecures use it too much.

As far as nixing the fantasy I wouldn't know how to advise you. The only thing I could think of is mindfulness therapy which focuses on paying attention to the here and now as well as your feelings. Both anxious and avoidant people generally lack mindfulness compared to secures. For me I have become very reliant on adopting some determinist thinking. My fantasy is that my ex and I get back together and live happily ever after. I have internalized that this is a fantasy that will never be real. I use things from our relationship to remind me but also I take a deterministic view of attachment theory. While I am sure there are anxious avoidant couples that work and are happy. I tell myself I won't be one of them. As soon as I identify someone as an avoidant I take them off the table romantically. Maybe I might miss out on someone that can control their triggers and has something to offer but I chose to believe the odds are against me and move on. It's possible you could do this with your fantasy partner. They don't exist, and any real partner will provided challenges and compromises.
If you learn of anything I am sure other avoidants might find it helpful.

I think the phantom ex/fantasy ex, is a psychological defense mechanism used by avoidant to create distance. I think the phantom ex refers generally to a previous partner they were in a relationship with that met a lot of their needs. This person and their reflection on the past with this person (often idealized in mind) become a point of comparison to criticize their current partner within their mind. They can never measure up to this person and thus they use this as a tool on a subconscious level to create internal and external criticism of their existing partner.

Both the phantom ex and fantasy ex become a dismissive weapon to keep their partner insecure and thrown off. My wife never had a phantom ex but always a fantasy or idealized ex. It was used to remind me often that I didn't measure up to her idea of the perfect person to her. It is a very effective weapon and it makes you feel like you are not worthy of this person and lucky to have them.

In regards to secure people with a fantasy sure I have have always had thoughts of what an ideal partner would be like, look like, and act like. But, it is more of just a roadmap for me in regards to what I am looking for in a person. I try to find someone with as many of these qualities that is attracted to me and that I am attracted to as well. Once involved I see less and less of the parts that don't meet my fantasy. It's not that they go away. They just don't effect me in a negative way and I find myself celebrating the things about the person that I like.

But, to your original question how do you just nix it... As a secure person this is difficult to answer, because what seems simple as a secure person seems very difficult for an avoidant. But, I think in order to do it you have to accept that your idea of a fantasy person is indeed just fantasy. You have to train yourself to look at all the good qualities in a person and not compare their bad qualities to your fantasy person. When you stop seeing the flaws you can see so many wonderful things in others. There is beauty in everyone but perfection in no one.

To me that seems very simple, but having experienced the other side of it first hand I realize how difficult it can be. If you can accept it and somehow nix it you are moving one step closer to secure. Which imho is a much happier place to be. I wish you the best on your journey and find the fact that you even asked the question a great first step.