I struggle with this as well. When I blunder I tilt hard and feel like the stupidest person around. A lot of comments are saying don’t equate chess with intelligence and that’s really good advice but it’s easier said than done. One thing I do is designate games in which I’m going to practice controlling my emotions. What I mean by this is that I will play a game and purposely blunder on move 10 (can be any move but close to the start of the game). Then I will take 30 seconds to a minute depending on the time control and just chill, making sure my breathing remains calm, and basically just practicing saying “whoops, oh well!” before the next move. I also force myself to finish the game because I have a bad habit of just resigning when I think I’m in a worse position. I know this seems really cheesy, but controlling your emotions is a skill, and you need to practice in order to get good at it. The goal is that remaining calm will become second nature when you blunder in a real game, and you’ll know it’s not because you’re stupid or even bad at chess, but simply you made a mistake. The more you practice this the more it will become ingrained in you and the less you will tilt. You can do this playing casual games if you’re worried about rating.