How should I deal with this disappointment?

In mid May we found out that the gig my boyfriend's dad got them tickets to for his birthday fell on the same day as our first anniversary. We had planned to spend the day together and have dinner and so on, and he's insistent that I go visit him so we can be together (he's 200 miles away), despite the fact that we will only be together until the late afternoon and I will be long asleep by the time he returns. So far I've acted fine with it and encouraged him to go because I just want him to be happy and to go have fun with his dad but really it's breaking my heart to think we won't be able to be together for very long or the way we planned and won't get to do anything special on our anniversary. I wouldn't want to go to the gig but even if I did take up the offer it'd be a risk to my health as I tend to have asthma attacks and pass out at gigs. I'd rather not go and to be honest I'm dreading the day because I know I'll be stuck in the house with his mother while he's gone and although we get along well it won't be a total barrel of laughs. This is not how I imagined for our very first anniversary - any other and I wouldn't be as upset - but I feel like I'm not allowed to be disappointed or sad and I don't have anyone to talk to about it. What should I do? I don't want to be selfish but at the same time this was the only day I asked for of his year. I'd rather he missed my birthday than this.