What if you never made a prayer of dedication to Jehovah? I bet they would have to annul your baptism for that. Here's what might work:
Go to the elders in tears saying you believe you've made a grave mistake. Tell them you fear you did not make a scriptural dedication in prayer to God. You were doing some personal study recently and came across some information in Watchtower literature about the steps involved in becoming a Christian and you read a description of what dedication means and it suddenly hit that that is not what you did. You thought you were dedicated because you didn't fully understand what it meant to be dedicated and so you thought you were dedicated based on your prayer relationship with Jehovah - you didn't realize that one had to make a special specific prayer promising to serve God unreservedly.
Act like you're devastated by the fact that all this time you weren't really fully right with Jehovah and ask the brothers to allow you the opportunity to be truly baptized the proper way after you make an actual dedication like you were supposed to.
Then you let others in the congregation know of your mistake. You start the rumor that all this time you were never really a dedicated JW like you thought you were and you'll be correcting that at the next assembly/convention. Get everyone accustomed to viewing you as a non-JW. Then when the day for your real baptism comes along, get cold feet. Have second thoughts about getting baptized and chicken out.