Very interesting. I'm in a very similar situation, is your mucus is yellow? I get it all the time but never yellow, it is always there though, sometimes I gag a little or cough. You might be in luck if it is because that would mean a temporary infection, that and smelly breath, that could be an infection or nasal polyps. People actually adjust to how much oxygen they get, so after a while it seems normal but you still feel like it could be better. I find exactly the same thing with Neti pots, its odd. Do you get a ringing in your ears or semi-bad hearing?
Don't believe its in your head, my doctor did for a long time, the bargain I struck is that if its in my head it'll be the last thing I look at, I'll look at every single physical possibility and if none of that works I'll have a go at looking at the mental aspect. I don't believe it is at all, I really hate the label depressed or anxious because like you I don't have anything that would actually cause it, you just end up chasing your tail if you pursue the psychological aspect and in fact making stuff up that actually wasn't there in the first place.
I like the way my post is now lit up with ads, that should be a give away as to how people try to sell you alternative medicines etc, pursue if you like but if you're like me you'll know what works and what doesn't very quickly. Neti pots and steam inhalations are definitely in the right ball park, they make at least some sense from a physiological perspective but if you're gonna dabble in alternative medicine try and read both sides before you go ahead otherwise you'll just lose loads of money on what doesn't work. I've been to a few and they'll often try to make you do it over and over again saying you need to keep doing in order for it to work...and it doesn't.
I wish you all the best because I've not found anything to get rid of it yet, I've not really met anyone yet who seems to have it as bad as me (I've met people like you who have to some degree) but they don't get a headache 24/7 and have massive trouble concentrating, breathing and just being ok.
Oh and if you don't believe any of this try drinking two pints of water in the space of an hour (or basically just try drinking a load of water) you'll feel better all of a sudden and probably have somewhat more concentration and energy (it is noticeable but its no miracle cure), this is likely I believe because it thins the mucus and makes it easier to breath. But if that happens you'll know that you have something similar to what I have.
Good luck
Whats a Zpack? I got this at uni when I started, did you? Its often a bad time when you get loads of illnesses. As I say you might be in luck, firstly because you've caught it early, secondly because you have many physical symptoms such as bad breath, headache just behind the eyes, post nasal drip and a set of existing allergies which will give your ENT or doctor something to work with. As I mentioned if you steer clear of talking about the mental side effects - irritability, tiredness, feeling down etc or if you just mention them briefly because then they won't focus on the physical problem which is the cause.