Have you tried having an open conversation with them? Tell them you're scared of losing them, talk to them about how much they mean to you and how you feel close and connected to them. It sounds like you are there for them and them for you, and I see no reason why the friendship should fall apart. A few years back, I met up with a person I'd spent 10 years talking to on here. Her anxiety is not as severe as mine and she's able to travel, so meeting her was inspiring, and we still keep in touch, and I consider her a good friend.