I feel like I let Gotye down, I feel bad about it now.
Seriously you know him? Why do you feel you let him down?
I was supposed to go to Grammy's together with him the day before but ended up not making it.
If you made a commitment but didn't follow through, you may have hurt the person you reneged on. You should consider doing something to make it up to him, or at least give him a sincere apology.
I can't believe I did that to him. I should have told him that I won't make it.
It's clear you feel bad about it, so just be honest with him and let him know how sorry you are that you let him down. If he is a true friend, he will understand and you guys can get past this.