There was some very helpful talk on here some time ago about the "holy boy imago".

The "Holy Boy Imago" is that perfect, pure image of the ideal boy that lives in our heart.

It is this imago that we long for, search for, (perhaps long to be) and ultimately worship.

When we see a real boy who resembles the imago in our heart we are attracted to him intensely. Even if we don't know the boy... just the sight of him... if he resembles our heart's imago... will make us fall over in adoration.

When our imago comes to rest on a real boy... such as the yf you now pine for.... we begin to think that the boy himself is magical and that we love him like no other, nor could we ever love like this again.

The reality is that no actual boy could ever live up to our imago.... but on the other hand... the real boy is wonderful (as a creature in God's image) in a way that our human imaginations could never capture.

So real boys feed into our heart to shape our imago and our imago rests on real boys making us love them.

For me it was so helpful to understand this truth. It helps me to separate my real yf from my heart's imago. It helps me keep my feelings in perspective. And most importantly, in your situation,... when the real boy is grown and moved on so that he no longer resembles your imago (except in your memories) you can know that it is not him you pine for but your heart's imago. That imago can and will come to rest on other boys... perhaps it will one day inspire you to love them as you love him.
But it doesn't mean you love him any less... you just love him for himself now... for your attachment to him... for the time you had together... and the rest of the obession is with your imago... something entirely different. Something you can work with in the here and now even if your yf is miles and miles away in space and time.

Blessings
Cat.