I do this when I'm manic, pretty much every time. I reach out and try to reconnect with people from my past, as well as try to connect with new people. When I'm manic - before it get bad - I'm super social and outgoing and fun to be around so I like to connect with people to go out and do things. But that's just not me. Typically, I'm more of a loner that doesn't have too many friends or people that I associate with. I generally enjoy doing things on my own so connecting and trying to reconnect with people is definitely out of the ordinary for me even though I'm surprisingly not on bad terms with most of the people from my past except maybe two or three people.
Like you said, you probably do it because of some nostalgia factor and because you have trouble letting go. If you are opening yourself up to get cut down, you probably should just let things go. Tell them "Hey, I'll be here if you want to talk or get together." to put the ball in their court. That way you know if they respond, they want you to be in their lives and if they don't respond, then you know they don't want to be friends with you. Talk to your counselor or therapist about it. And instead of trying to reconnect with people from your past, why not find something you could to to create friendships with new people?