I agree with all the other postings - we all make mistakes, the trick is to learn from them.
I remember a student at my flying club doing exactly the same thing. I think he flew a perfect circuit for runway 29, when the runway in use was 21 - which was even more spectacular since the circuit directions for the two runways are different. And this was at his home airfield too! I remember thinking I could never do anything like that. Then, a few months later, I was doing an American-style 45-degree join onto downwind, which was fine except I was joining for the wrong direction runway, turning downwind against the traffic in the circuit. It's the only time ATC have shouted at me on the R/T, and it was completely justified. So yes, it happens to all of us, even when we say it won't!
Advice about flying the approach in your head before you leave is fine as long as you know which runway you'll be using. You should always call first, of course, but there's nothing to stop them changing their mind while you're en-route. What type of join did you do? If it was an overhead join, the trick is to remember to overfly the numbers you want to land on before descending dead-side, then everything else fits into place. Other types of join are more difficult - get the airfield information as early as possible, use the DI to orientate yourself, and plan your arrival as early as you can.
And remember that we all start out with a full bag of luck and an empty bag of experience. The trick is to fill the bag of experience before the bag of luck runs out. You've just put another bit of experience into your fast-filling bag.
FFF
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