Feeling disappointed sometimes is normal. It happens to everyone. Just don’t make it mean more than it does.
It’s also pretty common to inflate the negativity in times of ordinary disappointment. We fail at or feel bad about one thing and start to think about our other failings, or even start to see ourselves as a failure. We imagine all sorts of discouraging consequences, based on not achieving what we want in one isolated case.
The feeling is common, but it’s a lot of baloney. It’s a waste of time and simply wrong. As long as you keep moving forward, you will fail sometimes and feel bad about it. Other times you’ll feel disappointed because of something someone else did. So what? If you have a tendency to judge yourself harshly when things don’t work out, imagine what you’d say under the same circumstances to someone else you care for. Would you judge them the same way? Of course not.
The only thing to do when you feel disappointed is to feel it honestly. It’s a sad feeling. Honor that feeling for a moment and it will break up and vanish. Don’t try to hide it, because hiding it makes it more likely to get stuck somewhere in your emotional system. Feel bad for a few moments. It won’t hurt you. Then move on.