Why is my Adderall XR not working like it did the first day I took it?

I started taking Adderall XR a week ago. My doctor put me on 20mg per day. The first day I felt really calm, almost comfortable in my own skin. Then the next day I felt pretty good until about 2 in the afternoon ( I take my dosage at 7am). After 2, my symptoms started returning, foggy, impulsive, and forgetfull. Along with a headache. My appetite has decreased, and I really noticed that I needed to eat more because my preformace at work was declining because I felt weak and disorganized. So then the next day, I made a not so smart decision, based on my dosage not working the day before. I took a second pill in the morning. I haven't noticed it being any better. I'm taking double what my doctor prescribed me and I really don't feel any different than I did before I started taking it. The only difference is I'm cranky now. I've tried a few different ways of taking my medication to like, eating before, not eating before, eating after, not eating after, not drinking coffee, drinking coffee, taking the second dose with the first, taking it later on in the day. It just doesn't seem to make a difference and I'm nervouse that maybe I have a tolerance, and that Adderall isn't going to help me. But it did in the past, and worked really well. My doctor mentioned in passing that she didn't want to put me on the regular Adderall, just the XR. Maybe the regular would work better for me? Any advice, or personal expierience feedback would be greatly appreciated :)