How Can I Stop Acting So Childish?

I’ve realized that my childish behavior is starting to ruin my relationship, and I want to change. I’ve been acting this way most of my life, but now it’s affecting the person I care about the most. I’m 20, and I know it’s time to grow up, but I’m struggling with how to start.

Here are some examples of what I mean by “childish”:

I painted my first-ever painting recently and showed it to my girlfriend (she’s a painter). She thought it was cool, and as a joke, I asked if she or her family wanted to buy it. She laughed at first, but when I later asked her parents the same thing, she said it came off as childish and unclear whether it was even a joke.

I make immature jokes about farts and other kid-like stuff.

Last night, my girlfriend asked me to bring up a movie website to stream a movie. Instead, I brought up a fast-food chain website with the same name, thinking it would be funny. She didn’t laugh and instead told me I’m acting very childish. Later, she admitted that my behavior is making her lose feelings for me.

When she said that, I got really sad. She noticed and apologized for being harsh, but also explained that she didn’t know how else to get her point across.

I don’t want to keep acting this way. I want to grow up and show her (and myself) that I can be mature.