This might not be what you want to hear, but you’re right. People will come and go from your life in ways you don’t have any control over. The only person who will never leave you is you. So, be a friend to yourself! You don’t have to be your only friend, but once you have a solid foundation of being ok with just yourself, you won’t need to depend on others as much. You’ll be better able to enjoy their company without worrying about being abandoned. It won’t hurt as much when friends leave, whether it’s intentional or just a result of changing circumstances. Of course, this is not an easy thing to just do. As others have recommended, a therapist could possibly help you with this.

I’ve been in the same situation you describe, and it’s taken more than a decade (and me having a complete breakdown and him still not leaving me even though I was a complete mess) for me to finally believe it. I’m guessing you may have had someone in your childhood who told you they cared about you, but their actions later revealed they actually didn’t. Exploring those feelings - the real abandonment you did experience - can help you heal and separate current and future experiences from the past.