> There is only a faint aftertaste of missing out, which sticks to me like tar. I can’t wash it off.
Sure you can. There are various paths to it, some outlined in sibling comments, and here’s another one: Pick up the phone and call or text some of those people you wish you had kept in contact with. Don’t have their contacts anymore? Ask someone who might or find them on social media. What do you say to them? “Hey, I was recently thinking of <that time you did something together> and felt like reaching out. How have you been doing?”.
Maybe beforehand “collect” some relevant events which have happened to your life since you last met, so you have something at the ready to keep the conversation going if you need. I’m not saying rehearse it, just have them in mind. If you need some small talk tips, see this short video.