Yes it is a sin. Jesus said that having lust is just as bad as committing a sexual act. It is the thoughts you are having that are the main sin. These thoughts are not of God, and can lead to demonic oppression. You should ask God what this masturbation is doing to you, and how it is going to affect your future relationships and your mind. Jesus died for our impure thoughts by wearing a crown of thorns.
Most people have committed this sin, but it can be somewhat addictive. If you can hate masturbation and the danger it represents to future relationship health, you are on the road to repentance and submitting every part of your life to the Lord.

) That is what it means to have a "circumcised heart": dedicating even the most precious, secret parts of your life to the Lord.
I'm proud of you for not looking at porn, though. I see that as a huge accomplishment. When husbands look at porn, their wives feel very rejected and ugly by comparison, because porn is lies. The women seem happy, but they are actually in pain and have been horribly abused, and the porn stars are always the same age, whereas real women get older every day. You have the right heart attitude, so keep looking to God and aspiring to the holiness Jesus demonstrated, while understanding we will never be able to perfect ourselves. We can only give up trying to be good and ask the Lord to change us. God bless you in your quest for holiness.