Try to apologize:

“I’m sorry, I was just so frustrated that you always treat Mona like she's perfect. And I thought it would be a way I could. I don’t know…” your voice trails off.

“Don’t stop. Keep explaining what you thought you were doing.” She said without turning from the road.

You swallow, “I thought it would be a way of getting revenge, violating something of yours,” saying it now made it sound way worse than your original intention.

“So you thought, the best way for you to show your displeasure was to masturbate while wearing my things? I don’t buy it.”

Is she in denial? “That's what happened. I’m sorry, I know it was wrong but-”

“Stop lying,” She said, pulling into the mall parking lot.

“I’m not! Why would I lie about that?”

“I don’t know, because you’re embarrassed by why you were really doing it.”

“What? That is why I was-”

“It’s because you’re a pervert.”