You raise an issue that affects more young men than you probably realize. When young men are unmarried their sexual drive seeks satisfaction, so you are certainly not alone in this struggle. Most Christians will agree that masturbation is sin for two very important reasons.
First, God has defined sex for within marriage only. The numerous Old Testament prohibitions on fornication or sexual immorality refer to any sexual experience outside of marriage. This would included self-inflicted sexual pleasure.
Second, most masturbation takes place with pornography to look at either actually or in your mind through fantasy. Since Jesus condemned not only the act of adultery but lusting in our mind, this is clearly included.
You must also keep in mind the addictive nature of nearly all sexual sin including pornography. It eventually becomes a form of idolatry. We worship our sensual pleasure over Jesus.
Jesus response to Peter's question as to how many times he must forgive (70 x 7) is meant to assure us of God's infinite capacity to forgive even habitual sin. Masturbation can only be conquered in the power of the Holy Spirit. If you follow Philippians 4:8 when tempted, you will find that the thoughts vanish or they remain only at your desire. It must become a question of Lordship: Jesus or you. The masturbation becomes only a symptom of a deeper need for intimacy with Christ. Habitual sin does not lead to questions of salvation but of Lordship.
I encourage you to seek first His kingdom and His righteousness and everything else will follow. This is not to say it will be easy or quick. True discipleship is costly and our personal secret kingdoms must be rooted out one by one. But Jesus said I will never leave you nor forsake you. He meant it.
Also, may I suggest two excellent websites for help with sexual addiction and pornography addiction? The first is Setting Captives Free at www.settingcaptivesfree.com, and the second is Focus on the Family's Pure Intimacy at www.pureintimacy.org.
Dr. Ray Bohlin