How to be friendly, concise and cool in turning down an invitation from your friends?

Assume that some of my friends invited me for an intimate gathering, but I have something on my hand right away so I can't make it. Now, the problem is that while this is a totally normal thing to happen, but I really wanted to be there! And at the same time, I don't want to write the reply(to their text message) in a fussy and long-winded way.
I know I could write:
I wish I could be there but unfortunately...
Or:
It could be great fun if I could... but unfortunately...
But these don't work for my situation. These are so cold!
I could also say something like these:
What a bad luck! Just today my manager handed me a new task and the deadline is so tight so...
This is a pity but I can't come I have...
But also these ones are a little too much emotional (We're boys).
If it was about anyone rather than my friends, it wasn't a difficult thing to put some words together and hit the send button, but in this scenairo I can't think of any "cool short and yet intimate" expressions. I wonder what are some of the shortest possible ways I can convey this meaning in a friendly manner?