Ok, sleeping with my bed fixer mat under the pad was a terrible idea. It's actually worse than my bed by quite a bit. It's firm, but not exactly intended to be slept on. It's intended to go under the mattress to stop it from sagging.
It's dense and doesn't have much give, or contour effect to it, but it has just enough give that it's worse than sleeping on my bed or on the floor. Every so often, I'd wake up with weird pressure points that ached, I'd have to change laying position, then go back to sleep. This happened all night long, and I got terrible sleep and a lot of lower back pain from it after just two nights.
So, now I've removed that, and will no longer be using it.
Extra note: There's a moisture barrier between my sheet and the pad I'm using. I was told that mattresses and foam wear out faster due to body sweat, which gets especially bad in the summer and on extra hot days. So I bought a moisture barrier to try and help the pad last longer. Didn't help my mattress last longer... but I've had the pad for a while now, and it's still good so far. Don't exactly have a control system to test whether or not the moisture barrier does anything, but eh. I'm not exactly made of money.