I don't know all that much about your meds., which may or may not be causing your HR difficulties. There are lots of different meds out there for ADD. You might also try green tea, I have a little bit of ADD and find that copious amounts of green tea, gives me quite a bit more energy and focus.
I do know that my HR is 145-185 when I'm running and that's very typical for my age, in fact that's pretty much the recommended range for a cardio workout for a 40 something female. That being said, how is your caffeine intake? You might try cutting back on that a bit and see if you even out. I don't use caffeine after 12:00 noon, because I have a heart condition and too much caffeine causes it to flare up. I also think you should talk to your doctor, and possibly see a cardiologist. My husband who is about your age has a heart condition that causes his HR to flare and it can happen anytime, but exercise and humidity almost always cause it to skyrocket and stay there. His situation is under control momentarily with medication but he will eventually need surgery to fix it.
You might also want to examine the labels on any supplements that you are taking. You would be surprised at how often my mother has to treat patients that come in to the ER, with a racing heart only to discover that they've been using some sort of herbal remedy, energy drink, nutritional supplement or they've ODed on Starbucks. So read your labels and do some research.
You know it's only human to backslide a little, and a couple of days off the workout routine is not a big problem in life. You wanted a better healthier you, well now you've got it, but in order to keep it you have to do at least the minimum maintenance almost every day. It's like having a pet really. The other thing I might suggest is that you might be a little bored with the P90X thing, which would be a typical ADD reaction since you've finished once already. So how about getting a few new workouts, to mix in with P90X, maybe something fun like the Wii Fit Plus. It's a lot easier than P90X, but addicting in it's own way. I've also heard good things about the Biggest Loser workout, maybe Jillian Michaels can kick you in the butt.