Well in the bible, sin is commited because you doubt the Lord and of Jesus to put you in heaven and so you sin to make you feel better or more well off.
Sin is divided into 7: Anger, sloth, envy, pride, greed, gluttony and lust. For me, I reckon the above are only sin if it affects other people in a bad way.
Since masturbation is really affecting no one but yourself, then i guess your forgiving. Hang on though, did you talked about it to other people close to you? If you have, were they concerned or smiled warmly? If they are indifferent or doesn't find it a problem then its not really a sin. If your actions (not of masturbation, but of talking about it!) influence others negatively however, then you should feel guilty.
i.e. Masturbation is neither sinful nor virtues if nobody knows about it. To talk about it, introduces others to it. Influencing others in a bad way = sin, in my book. It also introduces doubt and confusion towards the Lord's way of nature = sin, in His Book.
In short, masturbation is not a sin, is how it affects others that is sinful.