Oh, I use to hate it when everyone was going to go do something I didn't want to do so I'd say "No thanks" and then they'd go and I'd be lonely and wish I'd gone!

What helps me is concentrating on activities instead of the people. Have things of your own you're working on so it's a relief to have other people gone or away for a bit. A couple days ago my DH went shopping all day without me and to do chores and I did the same yesterday; while he was away I made my own list and it worked out great. If you are working on your own things what other people are doing doesn't make you so lonely.