Do you ever use your router for other template routing? If so, do you have the same problem when cutting them? Do you also use the jig for dovetails? Using the Leigh jig is basically the same process for dovetails or for the isolock joints, so you are going to need to figure out what exactly is causing your fear.
I found that waxing both the bottom of the router base as well as the top of the jig where the router slides, makes a huge difference in how easily I can slide the router along it's path. When the router moves easily, I find that I am more relaxed when using it. For the isolock joints, it's easy to fail to cut to the template 100% of the time on the first pass, so I always make two passes, one to clear the bulk of the material, and then a second time to be certain that I have followed all the edges of the template. I'll almost always find small areas that I missed this way. I can't remember having any more difficulty cutting isolock joints than when cutting dovetails on the jig. My biggest problem when first learning to use my Leigh D4R jig was forgetting to drop the guides back down after changing work pieces.
I think waxing and making the second pass will make a big difference in how well you cut your isolocks. Getting some more experience using the jig after you make these changes should make it much easier to use the jig and you should become much more relaxed with it. The more you do, the more your fears will subside and the more you will relax. It took me some time and use before I could relax when doing this too. Cutting isolocks or dovetails all day for a couple of days will make a big difference. You may also discover that you can't raise your arms above your shoulders after it too. I ended up building a wood platform the same height as the jig, with a large hole in the center, so I could place the router easily on it when making work piece changes or adjusting the jig.
I now also use the lightest 2 1/2 hp fixed base router that I have, a DeWalt 618, with the D handle fixed base, when using the jig. The D handle helps me control the router better than the standard fixed base.
Central North Carolina