This might not pertain to you at all but I struggle with something very similar.

My T believes that when I attach to someone, I am looking to fill some void that I'm not able to (or don't know how to) fill myself. Something triggers me so that I somehow feel inadequate and feel like I need someone else to validate me (tell me they care, that I'm important, that I'm not alone, etc.).

So I guess to answer your question, I think that the way to stop wanting what we can't have is to get it from somewhere else. We don't have control over other people but we do have control over ourselves so if we are able to help give ourselves those things we want (someone caring about us or being there for us), then we don't necessarily crave that so much from other people.

I've spent many hours with my T talking about self-compassion and I'm not fully convinced it can completely solve this problem but it can definitely help. Being able to love ourselves, care for ourselves, and validate ourselves can help alleviate the feeling that we need that from someone else.