I'm going to start this opposite to what I normally do. The answer: Stay true to yourself.
It may come to a surprise to some people that you are a very real and genuine person. People will question you if you are that "nice" person, because it is almost "impossible" for anyone to be like that. Unfortunately, that question of someone's and or my, character is quite hurtful.
I had to stop myself and think what does that matter? Why should I let their opinion of who I am affect who I am? The simple answer is, that it shouldn't.
It is easy to get lost in the opinions of others and let that define you. But, you know who you are. You may be still searching or even changing--it doesn't matter, whatever it is, you still know who you are down to the core. It is a lot of effort to not be who you are.
So simply saying, if you are one of those people who get into moments where people think that you place this mask on when you are not, forget them. Forget those people, because they do not truly know you. If they did, they would not be saying anything like that in the first place.
We may question ourselves at times; have our doubts--but that right is reserved for us only. It is up to no one else to decide that for us. You are who you are and that is enough. Be your real true self, and don't let anyone dim that light you have going for you.
If anyone knows you better, it will always be yourself. Never ever forget that. Don't let anyone make you think twice about what they assume, because we all know that people who assume make...(well you know the rest).