Hello Winglets and welcome to the site.
I do wonder if you are overthinking everything and worrying un-necessarily, especially as you have anxiety issues.
For a start, mirtazipine can itself increase urination so that may well explain the extra urge, plus when you are aware of it, the need becomes apparent.
During the day you get on with your life and probably don't think about going off for a pee if you are busy, it's when we are in that half sleep that we think about it and the thought becomes the deed.
You may say to yourself that now you're awake you might as well get up and drain off so that you don't get disturbed later in the night. Constantly thinking about it makes it happen.
I'm sure your GP will have checked you for infection (urinalysis) and I am also sure that he will know if en enlarged prostate is apparent.
Please try not to be unduly concerned.
You are an otherwise fit and healthy young man. Don't waste your youth worrying about whether you have something wrong with your prostate.
If it will put your mind at rest I suppose you could ask your doctor for a PSA test but I think he would be likely to refuse due to your age and no other symptoms. You haven't mentioned any family history apart from dad being diabetic so I assume not.
IT would seem to me that you are actually causing the problem by focussing on it.
Anxiety can make us susceptible to all sorts of thoughts and personally I think it's your anxiety kicking in.
Check up side effects of mirtazipine and I think you'll find that it may be the root of your problem.
Don't let it spoil your Christmas
We can't control the winds - but we can adjust our sails
Sounds like bladder retrsining might help - once it becomes a habit, your subconscious takes over and rmbeds the behaviour
"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard