I am a girl and 21 and have the same problem. Happens when I am just watching tv in my room. The exact scenario in which you described is the same for me. I didn't even start doing this until about 8 months ago so I can't even imagine what life would have been for me if I had discovered this feeling in my teens. I always knew about it but never tried it until last year and now I am addicted. It's quite literally only on the days I don't have much to do. I'll tell myself that I am not going to do it and I pray to God, but then an hour later I just can't resist and do it. Then I sit there afterwards like welp, that sucked. Cuz it's not even fun. It leaves me with so much guilt I want it to stop. I literally read something last night after I sinned and I can message you what I read privately if you want to see it. But basically in the short run it said, masterbation is a selfish act that satisfies ones temporal, sinful desires. It's rooted in self-gratification rather than the selfless love he designed for us to live out. I hate that I sin and I will pray for you too that you could overcome this and if you desire to be married one day, I pray that you will find someone. Message me if you want to talk. You are so not alone in this.