XL (extended release): take once a day
SR (sustained release): take twice a day
Regular (immediate release): dosing varies, but generally it would not be prescribed just once a day, and one would work up to 3 times a day, either 3 x 75 mg or 3 x 100 mg. It is often used initially when figuring out the right dose for the person. Then a switch is made to SR or XL.

I have taken Wellbutrin for several years and really like it. When I asked my prescriber about the med causing anxiety, she said that there may not be one med that would help with all my issues, so the Wellbutrin might help with my ADHD and depression, but not anxiety. So I shouldn't have expectations for a cure-all. A prescriber has to start somewhere, and ADHD and depression is where she chose to begin. My experience was that the first few days on Wellbutrin were super activating but that went away soon. Whenever I had a dose increase, the same thing happened. So if you do feel increased anxiety at first, it might resolve. I now only take a low dose--100 mg SR once a day--and this is enough for me. (Helps with ADHD in a way that a stimulant alone does not.) But I used to take as much as 300 mg. My only side effect at that dose was hand tremor and I just tolerated it rather than switching meds.

Good luck!