I’m 33 now, I don’t remember my sleepovers and all of my highschool friends are gone. We see each other every now and then when it’s convenient, but the new friends I made late 20s are the people closest to who I am now.
You aren’t “missing out” and feeling like you are is only going to make your confidence issues worse. High school is not what defines who you are.
If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment. (Markus Aurelius).
You have the power to feel confident by altering your estimate of pleasing people. Please yourself, confidence and everything else you feel you want will come much easier. Good luck!