Hi Sledgehammer,
It seems clear that one 15 mg tablet by itself will not put you to sleep. That's compounding your anxiety - you actually seem to be more anxious in bed than out of it. Plus the mirtazapine is making you drowsy that lasts for twelve hours or more.
A couple of issues here. The daytime drowsiness is a start-up side-effect which usually disappears after a week or so if you persist.
Furthermore, increasing the dose can actually reduce drowsiness, but still help sleep. It's worth a try. You can go up to 45 mg a day.
Finally, mirtazapine doesn't always work well for anxiety, so it may useful to try / add something else, like pregabalin. I had really bad sleep problems - multiple awakenings per night with jolts of anxiety, but a combination of mirtazapine (45 mg) and pregabalin has me sleeping well. However, as Keggsy says, some people just sleep worse than others, and it's not a cure-all. Good luck.