Does taking a stimulant make my self reports of induced positive affect invalid? just wondering.

I have to say that I was in an overall funk today. I used to be really prone to letting this consume me. I am much calmer about it these days and am better at reminding myself it’s not a permanent state of mind. Well it was almost time for class and I’m thinking after being calm with my funk since 7 this morning, “Dang, this is really sticking.” What an opportune time to do our blog assignment. I ordered decaf coffee so I wouldn’t OD on caffeine because I really enjoy dark chocolate covered espresso beans. This was one of my positive mood inducing tools. I hung out and did some reading for class, which I actually enjoy doing so this was intended to add to the educed positive emotion. So, the test came with class. I know it can be attributed to the caffeine before class but I definitely think being conscious of the fact that I felt like crap before hand and about wanting to get out of it set me up a little better for class than if I sat and did my reading and just wholly excepting as fact that I was going to feel like shit for the entire day. I was surprised when I left class because I really came around on my way to my Organic Chemistry Review, which I was definitely not looking forward to. I don’t ever mind going, but I went from simply excepting the fact that I was going to my review after class to being glad and getting a lot of pleasure out of understanding the material and feeling good about knowing I’m preparing myself to do well on the tests.
I know myself well enough to know I get a little euphoric with caffeine in my system. Despite this I do believe the attempt to induce the positive emotion did some good in addition to my lovely chocolate covered espresso beans. I enjoyed my ride home and did a few things, including this assignment with a little more enthusiasm and enjoyment than I think I would have without choosing to be a little more positive.
If I did this on the regular? My initial response to this question is me getting am image of myself eventually exploding from all the thwarted emotions, but I know better. Like anything someone chooses to master this is probably a very beneficial skill to have and practice.