You can't always be as generous as you would like to be with your friends. When you don't have the time, you don't have the time. Trying to convince yourself otherwise is only going to make you resent them for forcing you to squeeze your already tight schedule even tighter. Don't make promises you don't feel comfortable keeping. They will respect you much more if you say you can't help out than if you say you can and then just flake out at the last minute.