Ah - the old "my homebrew has a HOMEBREW flavor"... I fought that battle for over 6 years. I too know the frustration of trying harder the next time - only to be completely disgusted with the end result. I too changed my aluminum brew pot, water source, yeast, LME to DME, fermentation temps, sanitation, etc - the list went on and on. Over the last 2 years since becoming unemployed - my beer if finally BETTER than the commercial equivalent, and all friends and family actually look forward to coming over and sharing a homebrew (as opposed to being polite). I kept maticulous logs - and I feel a few techniques I learned here have improved my brew - but the #1, hands down, difference I made was going all grain - really. Before the others jump in - YES, you can make good beer with extracts - but obviously it is a struggle for me. My beers instantly dropped the "homebrew" flavor on my first all grain batch. I went all grain to save money per batch - and it turned out to be the best decision I have ever made. I am now proud of my work instead of dissapointed.
A few other smaller changes I feel helped:
Lower fermentation temps - especially early on
Do full boils
Let the beer sit three weeks in the primary instead of two (I also do not secondary)
Don't give up, every failure is experience to help you make much better beer (and it makes the successes much better!).