I can definitely relate to this. Especially when it comes to ex's. There are so many layers and people on here have already given you some great responses so my thoughts on this may not add much, but here goes .

At a basic level, obsessions are thoughts that get reinforced. I think what's important isn't so much the content of the obsession or the fact that you're having those thoughts in the first place (it's easy to beat yourself up for having these thoughts, isn't it), but it's more the meaning that you attach to the thoughts/obsession/fantasy that have an impact. So it isn't just 'I'm thinking about my ex constantly'- it's important to identify what it means to me - am I thinking of how I deserve to be punished for something bad I'd done in the relationship? Am I thinking that this person would never be with ME (insert all the negative things I think about myself)? It is easy for thoughts to take a negative downward spiral (for me at least). Negative self-talk is a big part of depression, as well as hopelesness - I think sometimes obsessing about someone who isn't available sets up this cycle. Ask your brain a question like "why aren't they with me?" "what's *insert person in their life* got that I don't?" and your brain will fill in the gaps with a lot of negative or insecure answers. Most of the time, our feelings are related to the assocation or perspective we take on something. I hope that all made some sense ...

Obviously this person was very special and was a very positive influence on your life. I liked the idea someone put about how it's likely filling a void. Sometimes we meet these incredible, influencial people and it's only natural to want that feeling or that person to be part of our lives. I think it's natural to feel a loss or a void when we either can't or no longer have these people in our lives.

Thoughts can't be turned off, but constantly reinforcing them makes it more likely that they'll keep showing up (i.e., by attaching meaning). One thing that helps me is to not lock on in an emotional way to my thoughts. It isn't always possible of course! Distraction is helpful to find other things to put your attention or energy on to. There can be room for other things besides thoughts about this person. Channel the positives you took from your experience with this person toward a new direction in your life. The person will never truly be gone and in a way you're preserving the good that came out of that relationship and applying it to your life now. Letting go doesn't have to mean forgetting.