You're getting a lot of good advice, but it sounds to me like you need to focus on the basics. Most notably this:
Originally Posted by carvedog
Have you tried T-ing up to the waves?
Try and hit the waves at a 90 degree angle. IKs are super survivable if you hit everything straight on, but if you get turned sideways they'll flip pretty easily. Also, you will generally want to be actively paddling. That will give you some downstream momentum, and each paddle stroke will act as a mini-brace. As you get better you won't need this so much, but it helps when you're a beginner.
A wider boat with bigger tubes will let you get away with sloppier lines and technique, but there's no reason you can't do class IV in the boat you bought. A different boat might make it less frustrating to learn though.
I don't know Bull's sluice, but I just watched a couple videos. It looks like it's basically one drop? The left line looks pretty straightforward - just hit it straight. The right line looks a little trickier, but I'd probably try and hit the drop with a bit of right angle and momentum and be ready with either a paddle stroke on the left to counter the water pushing you to the left or with a brace on the right and let it turn you. If it's really kicking your ass, I'd just walk it until you get better. Someone suggested watching other people's lines and mimicking the best ones. That's good advice.