My close friend's 20th birthday is next month and on her birthday we're all going for a meal then on the Friday after she's organising a night out. I have a chronic illness and can't go for a night out which she completely understands.

I've just realised though that a concert/gig I really wanted to go to is on the same day as the night out. It's right next to my uni accommodation and starts 19:30 so I think I'll be able to go to it BUT I'm worried she'll think I chose it over the night out.

I just want your opinions. How would you personally feel if your friend couldn't come to your birthday night out because she was ill but went to a concert that same night?

I know my own limits and I know that I would be able to do the concert but a night out is too much buuuuuuuut my illness isn't well understood and even though she's my best friend she doesn't understand it. Explaining isn't really an option either because I've tried explaining my limits before and she doesn't get it - I don't blame her, even doctors don't get it. I don't want her to get the wrong message if I go to the concert :/ but I really want to go I just don't want to hurt her feelings.

Thoughts?