Masturbation is sinful. I've been doing it for the past nine years and have been with this guilt all along. Now I can hardly pray cos I feel so guilty . Am about to get married. What do I do to stop this? Someone told me that once am married, it'll stop since I already have wife to have sex with. Can God still forgive me given that I've abused his grace time and again? I even feel too guilty and ashamed to pray. How do I stop this bad habit? I need help. Anyone with experience and matured responses will help. Thanks.

SS is a pointer to needs that your soul has that the flesh tries to fill, which it (the flesh ) cannot do, hence the feeling of emptiness within & guilt after each indulgence.
First off, come real with God, stop seeing Him as a distant holy king that only the blameless can reach, NO! He is reachable to the vilest of humans. Let nothing hold you back from Him as He is not surprised or put off you because you SS. A part of the Godhead became flesh remember, so God knows all the passions we feel as humans so feel free to tell Him all your struggles in this aspect, a million times every second, every day if that is the frequency you SS.
Then make sure you are filling yourself up constantly with quality soul food at every opportunity you have, not necessarily by moving your things into a church building to live there! There are so many books and online material that deal with spiritual growth that can help you.
You will also do well to identify your 'triggers' and consciously get rid of the ones you can remove and avoid the ones you cannot.
The result may not be immediate, even when wife comes, but as you faithfully fill your spirit and mature over time, all those passions of the flesh will starve to death & fall off on their own accord.

I have been there! Cheers