The first version just tells us that you regret NOT GOING to the party. We don't know why you didn't go, but the assumption is that you chose not to. Perhaps you were feeling tired or lazy, or you thought that it wouldn't be fun, so you didn't go. You now regret that decision. The second version gives more information : it tells us that you couldn't go to the party. Perhaps you had to work that day, or had another engagement that you had to go to. You're suggesting that you would have gone if you had been able to. In other words, you regret NOT HAVING BEEN ABLE to go to the party. This is something that you might say to the party host when you're speaking to them after the party - you're saying that you're sorry that you weren't able to attend. NB The choice of the word 'come' (rather than 'go') suggests that you're speaking to the host of the party. This makes the second version a more likely comment : your excuse for not attending your friend's party was that you weren't able to go (version 2) rather than that you chose not to go (implied in version 1).