I wouldn't worry too much about it. No matter how bad you think you are, there are people out there a million times worse. There is nothing wrong with rewarding yourself for your hard work with the things you want. The only exception is if it caused your family to have to do without. An example would be, "Sorry kids, but we won't be going on vacation this year, because I spent $14,000 on guitars."

If you are truly worried about it, here is a free evaluation:
You have a compulsive personality. This isn't a bad thing. People with compulsive personalities are some of the most successful people in the world when they focus their compulsion on productive ventures. The down side, is that compulsive people that focus their compulsion on negative ventures (smoking, drinking, drugs, porn) end up letting their compulsion get in the way of productive ventures.

I wouldn't classify your compulsion as negative, unless it negatively impacts your life in some way. Are you missing work to go guitar shopping? Can you not walk through a room in your house because of the musical gear you have piled up? Is it causing problems with your marriage? (Sometimes you need to keep the guitar, and get rid of the wife) If the answer to these questons is no, then your compulsion is not getting in the way of productive ventures, and not a problem.

Sounds to me like you think you have a problem that you don't really have. Based on the language you used, I believe it's based in guilt. I believe that you enjoy, as you should, being able to purchase the things you like. When you use words and phrases like:

"Materialist"
"More than you need"
"More than you should"

it's making a judgment about how you think other people view what you have. You should not feel guilty about being able to provide yourself with the things you like if you have the means.