Hello Vivianne, welcome to the site!
I've been thinking hard on this question, and it's indeed very tricky! I think the biggest reason someone doesn't step outside of their comfort zone and listens to what's inside themselves is fear. Fear of failure, fear of judgment, fear of mockery and so forth. If we stick to the things we know, we're playing it safe, even if it means we're not expressing ourselves the way we want to.
Letting go of this fear is one step to get your inner voice out, but it's easier said than done. Sometimes we have to just go with the craziest ideas to make it happen. I did some research on how to help you with this issues, and came across a couple of resources. Maybe these can help you out?
http://www.wikihow.com/Think-'Outside-of-the-Box'
http://www.shutterstock.com/blog/5-effective-ways-to-think-outside-the-box
https://www.psychologytoday.com/blog/the-power-forgetting/201404/surprising-exercises-get-you-thinking-outside-the-box