What doesn't challenge you won't change you. No pain no gain is not just a cliché – it's science. Your body will adapt to whatever stress you put on it - it's very efficient. When you start a new fitness class your body is put into a state of shock and very quickly sets about recruiting and building the muscles necessary to deal with the class. However, eventually, the same class will not challenge you in the same way. You'll get really good at and look great doing it...perfect form etc, but you will not see the same benefits as in the beginning. Your body has adapted so well that it is used to the class and has no further needs for adaptation (ie muscle growth or increased cardiovascular strength).
If you're not continuously progressing the exercise whether that's by adding weight or reps or using less rest time...you will plateau and you will get very little out of the allotted time (other than calorie expenditure). So why waste your time? Why kid yourself? Go grab a lemonade and enjoy the sunshine instead.
Being honest with yourself is a huge part of becoming better, fitter and stronger. It's all too easy to attend a class, sweat a little bit and pat yourself on the back for going to the gym.
I'm not dismissing the act of getting yourself in. It can be hard, but that's normally the hardest part. Once you're in...give it your all. What's an hour of discomfort in the grand scheme of things? So push yourself. Fail at the exercise. Try again. Try the class you think you're not good enough to do.
This is how change, progress and growth are really made.
And I promise you, that attitude will spread to the rest of your life. You'll work harder for the things that you want because you know you have the determination and willpower to do so.
You'll keep fighting even when others give up because you know how to survive outside of your comfort zone. And nothing will seem impossible because you've already achieved so many things that you once believed you would never be able to do.
Don't punish yourself in the gym. You've done nothing wrong. Have the right attitude and come to the gym to challenge and change yourself.