Unrequited love is agonising. You have my greatest sympathy. I've experienced it for the last 2.5 years and it's literally brought me to my knees with pain...
...and helped me transform my life.

I agree with other posters who have talked here about brain chemistry. I've read a lot about seratonin, dopamine and cortisol and it's helped enormously.

But great revelations and change occured after I began to read books about co-dependency, committment phobias and issues, and began to see that yearning for an unavailable partner might be a sign of my own issues with intimacy and committment (Steve Carter & Julia Sokol: Getting to Committment was an awesome and transformative read for me, as well as Melody Beattie's Co-dependent No More).

I worked so hard on myself, to be self-compassionate, self-championing, self-protective, deal with my childhood abandonment issues, pay attention to my brain chemistry, and get into mindfullness - I began to change. I began to focus on myself rather than him. I dealt with the pain and felt able to breathe and live, after 2.5 obsessive, agonising years. It's still wobbly. Challenging - every day. I think of myself as 'in recovery' rather than 'recovered' - and the story gets far more complex (that's why I've just joined this forum), but what I want to say is that it IS possible to recover. Not easy, and not fast, but possible. And you can use it as a way to transform your mental health and your quality of life.

Good luck! It's so good that we're not alone...