My biggest regret is that where I live, it isn't easy to sell your regrets . I had to give away my digital system (2 bodies, lenses, flashes) as it just wasn't being used and I didn't want it to break so I gave it to someone who needed it a lot more than I did.
For me, I came to the conclusion last year that photography is my hobby, and it gives me the most joy other than family and friends, so I don't need to justify spending on it once I can afford it (buying something and affording something is 2 different things). I gladly make a lot of personal sacrifices to support my lifestyle, and though my camera collection is only 3 cameras (4x5, 6x7, 35) - all of which were bargain basement prices when I got them; I do spend a lot more money on props, lighting, grip, film and processing than on cameras (thankfully I have all the lights and grips that I need, but film and processing costs will unfortunately keep rising).
Some people spend money traveling, some on clothes, some on gambling, some on drugs, some on their cars, some on their houses and we choose to spend ours on photography; as long as you can meet your obligations and no part of your life is suffering because of it, and you enjoy it, then there is no reason to feel guilty about anything. Life is just too damn short for that!