I'm 15. I'm sorry to disturb you, but I have a problem. I just can't stop thinking about sex with girls and boys. Every time I study, I just start thinking about it. Whenever I'm doing anything, it's coming. I'm trying to resist, but I just can't. Please help me.
Have you been looking at pornography? If so, that would explain why it is consuming your thoughts so much. See: Lies Pornography Tells Men. Looking at pornography has to stop immediately since it is a sin. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5).
These thought bother you most when you are bored or have nothing particular to think about. The solution is to get up and move for a bit to break the cycle of your thoughts, then focus on a task that needs to be finished. You will just have to force yourself to stay on track. Each time you get off, move around, exercise, run around the block, etc. then come back to work.
I stopped watching porn a few weeks back, but I always feel I've sinned when I think about sex and resist thinking about sex. Is it a sin?
It will take a while for the images you've seen to start fading in your mind.
You are growing older, and a part of that is your body develops a desire for sex. That is normal. However, Satan uses that desire to encourage you to satisfy it in sinful ways. That is temptation. Being tempted is not a sin because it is something being presented to you. If you accept the temptation and start thinking about how you can do the sin and get away with it, you moved from temptation to lust. Lust is a sin because it is justifying sin in your mind.