I was almost in the same boat as you were when I first started tournaments. Not quite as bad but still getting my butt handed to me. What made me turn it around the fastest was hating to lose. I see people all the time that accept losing. Me I say NO WAY! I do not accept losing anymore it is not an option if it happens it happens but I am not happy about it. A good fisherman around my parts told me once that you have to hate to lose more than you like to win. Everyone likes to and wants to win but there are some that hate to lose more than others and that's what counts. I started taking that approach and it has worked. Secondly nothing can replace time on the water and I mean nothing. Time on the water is crucial to success as well as that killer attitude.
Do not think prefishing is a waste. In summertime is when bass especially largemouth are the easiest to pattern. However, don't get caught up in I caught them here yesterday FISH THE MOMENT. Let your instincts and experience guide you through the day. After a while you will be amazed at how you make decisions on the fly that more times than not lead to success. This though comes with that all important time on the water. Summer bass usually are consistant and easier to catch.
Stick with it do not give up. Remember "quitters never win and winners never quit"