I started play testing Pandemonium Estate long before it was even ready to be play tested. I was eager to play because thought I nailed the mechanics of the game on my first try. I had a group of my good friends play the game which, at the time, was very roughly cut out from white board and used paper cards. No shame there, all games start somewhere. I remember how excited I was after the play test, but I couldn't have been further from completion.
Once I wasn't playing the game with my understanding and forgiving group of friends, I started taking it to various game stores to be tested by strangers. That's where things got tough. At this point, I had only been working on the game for about six months, so the prototype was still quite disheveled, but I was proud of it. And I knew I had a good game....or did I?
When you create anything and then have someone give you feedback, there's always that initial sting when they tell you that it's not as good as you thought it was. We have a special connection to the things we create. As created creators, we are invested deeply in our concepts and take it very personally when someone is apathetic. However, being able to overcome your ego and take the criticism will carry you a long way with game design.
The creative process in a nutshell.
You need to quickly learn what is constructive criticism, and what is just an opinion. Pandemonium Estate is a take on traditional "Roll & Move" style games, therefore, the elitist nerd who only plays 5-hour war games with miniatures probably isn't going to enjoy my game. The key is finding people to play test your game who play all sorts of games, and are open-minded enough to look at it objectively and give you good feedback about the mechanics, theme, and overall enjoyment.
It's not so much the criticism that is discouraging, it's the judgemental picking apart from opinionated and biased gamers. Once you are able to sort through the judgement and consider the criticism, things become a lot easier. The best tip I can give you is this; let people say whatever it is they're going to say and acknowledge their feelings. Even if they're completely wrong and you don't understand where they're coming from at all! Do not argue with people's opinions of your game or try to defend why or why you didn't do something unless you're asked directly. Write everything down or take mental note, then go over all of it later. Disregard any passed judgement on your game that doesn't fit your market or theme, then sort through the criticism and consider applying changes to your game.