"So whatever you are fantasising (Did I spell that right?) about you are essentially committing adultery with."
How can it be adultery if neither you nor the person you are fantasising about is married????
Jesus said that if you look at a woman (I read a real life particular woman) and think about sex with her then that is the same as adultery (obviously assuming one of you is married - otherwise He would have called it fornication surely???). His concern seems to me to be the betrayal of your marriage covenant... not the fantasy so much. Maybe it is both as so many people seem to take it. I'm not so sure.

What if your fantasy is not about a real person?... but some made up one?
What if you fantasise about having sex with yourself???? I've done this. Imagined the adult me having sex with the 10 yo me. Is that sin???
Oh look I'm lusting after myself! (excuse the sarcasm).

"Now, I used to feel very guilty for the times I failed. But what can I do? Biblically speaking, I can't have sex outside of marriage. And this sexual frustration becomes to much sometimes and I need a release. I'm doomed right? Not at all!! Thank God we are Justified by "Faith" not by "Works" lest any man should boast!"

See the problem I have with this line of thought is the idea that we ARE going to fall but its ok cause Grace and forgiveness and faith will some how see us through.

Whilst I beleive this is true on one level I also believe that it is incorrect to think that there is sin that we CANNOT overcome. If we say "I can avoid masturbation most of the time but inevitably I'll do it and I can't NOT do it" then God has left us with a sin we cannot overcome and this is clearly not Biblical. "shall we continue to sin that Grace may abound... Certainly NOT" (Rom 6:1-2). "I can do all things through Christ who strengthens me" (Phil 4:13)... "No temptation has seized you except what is common to man. And God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it." (1 Cor 10:13).

Thus, I can only see two options in the light of this:
a). Either we CAN win a total victory over masturbation.
or
b). Masturbation is not a sin.... but the normal healthy response to a natual God given body cycle.

Now how fantasy features in masturbation is another issue again.
The question there is can we learn to masturbate without fantasy?
If the answer to that question is NO... then see a and b above.

Blessings
Cat.