Relationship advice please?

Hi all, my SO just said to me that he's sick of me being either too immature or too serious.. that there is never an in between anymore. How do you not take life so seriously, yet not be too childish? I grew up in a house and environment where there wasn't much fun around and I didn't get taught all of these things. I'm pretty upset and not sure what to do. I feel kind of hurt but know what's been said is true. 😔