15 years? That means this relationship started when the two of you were only 10. No wonder he is so important to you! You saw each other through the ups and downs of adolescence and early adulthood.
It’s possible your friend realized that an unhealthy lack of balance in your relationship had developed over time, but he couldn’t figure out how to revise the terms of your friendship any way except to leave it. It’s sad he couldn’t talk it out with you. I’m sure it has felt like a betrayal and a huge loss. The way he left leaves it to you to find closure on your own. I hope you can find a way to honor the long friendship you had while also acknowledging that growing up sometimes means growing apart.
It’s likely that the exclusivity of the relationship meant that you didn’t develop the social circle of good friends that you need. You spoke of acquaintances, not friends. I hope you can develop some of those relationships into a friendship circle.
Yes, you’ve been left with a hole in your life but such holes are also opportunities. Your friend was never going to become your partner even though he was filling your time and your emotional life.
Please make yourself available and start dating. Find a man who isn’t looking for a personal therapist but who can love you and cherish you from a position of equality and respect, not neediness. You deserve to have such a man in your life.
I wish you well.