First, unless you are about 85-100lbs, I would crank up your speed by at least 2mph, or else when you drop into the landing you are just going to sink to your knees, and make this trick a lot harder. Without having a video to know for sure, most of the time when people end up going 180 on trying a tantrum its because they are not squaring up enough on the take off. You can fix this a couple ways, one is to fully let your arm all the way out when you do the trip part of the take off. Most people's initial reaction is to keep the arm bent, its ok to have a slight bend 10-15 degrees, but any more than that when your learning will throw you off. The other way to fix this is to do more work practicing back tucks. If you have access to a trampoline, or even the back swim platform practice using your hips and your knees to help start your trick, the more you are use to flipping, when you convert it to the water muscle memory will take over and help your board be forced in the right direction. When you practice, try to focus on watching your landing. As you start to do more inverts you will be amazed on how you will land where you are looking.
See if you can post a video from a phone or something, it will help people give you advice, and for you to see what you are doing.