I'm afraid to return the underwear I used to masturbate with when I was 13
This sound odd and makes me feel rather uneasy. Is there some kind of reason behind you wanting to return the underwear?
At least I think so. However, I have no idea why I want to do that.
It is bad to take someone's property and use it to masturbate since it soils that property. While it is good that you want to return the underwear, it might not be possible to do so since it is dirty and tainted by your actions. You should stop stealing undergarments or otherwise you will be found out and get in trouble.
You sound like my parents when trying to get me to stop masturbating.
People do not usually take each other's property for their own use no matter what they use it for. I think you should pay more attention to your boundaries with others.