It's hard and we've all experienced the same kind of disappointment. I was invited to be apart of my University's Model United Nations delegation and had to turn it down for the same reasons. It killed me because the team's adviser invited me out to lunch and begged me to join after he read some of my Poli Sci papers. I trust that just like that opportunity was placed in front of me, God has bigger and better ones out there coming my way soon. I believe the same is true for you too. Try to stay motivated and not let it put you in a dark place.