The bible story that is most often cited as the "word of God" making masturbation as a sin is that of Onan. See Genesis chapter 38.
The story basically goes that Onan had the duty to help is now widowed sister-in-law produce a male heir, because...reasons. Rather than help her produce a child he pulled out and "spilled his seed on the ground."
He was commanded by God to make a baby but he was disobedient. His sin was not ejaculating on the ground but deliberately NOT impregnating his sis-in-law.
He didn't masturbate, he disobeyed the commands of God. His wasn't even a sexual sin since he had the command from up on high to drop his seed in her fertile soil. He chose not to, hence God killed him.
I know what I believe on the God is/isn't real but I'm not even going to get into that. If someone is telling you masturbation is a sin because of this story you can totally tell them they are wrong!!!
As a licensed professional counselor, trained and fully licensed to provide mental health counseling services, focusing on adolescent issues (most specifically substance use disorder counseling) I say that the guilt that's being placed upon you, either from parent/pastor/minster or from yourself, is way more harmful than the self abuse you're being accused of. I would hope that you can find a way to not feel guilty about an act that is pretty natural and in some ways physically healthy for your reproductive parts (consult a more knowledgable source for this point though, like a urologist). More importantly you shouldn't let this guilt be the thing you beat yourself up over. If this is a thing that is effecting your mental health, PLEASE seek some professional help. You can look for mental health providers in your area online very easily. Try a school counselor, a trusted but not biased person from your church, or psychology today's therapist pages on their website.
As a person raised in a southern baptist church I do sometimes hate the way western christian churches can focus more effort on keeping a young man from touching himself than actually doing what God commanded and taking care of our brothers and sisters.