Brief Answer:
Recommendations

Detailed Answer:
Hello and welcome,

This sounds pretty rough. If you can feel your sinuses cause drainage down your throat, I doubt that the problem is acid reflux from your stomach.

The Flonase is not a bad idea as it can help stabilize the mucus membranes, making them less reactive and drippy, but it may not be enough.

The drainage, if it is not from an infection (which I will assume has been ruled out by your doctor), can be due to allergies, or to irritants.

If there is the possibility of allergies, do what you can to minimize your exposures to these, especially in the bedroom, as you likely spend 8+ hours there, and that is a long exposure.

Wash bedding weekly in hot (140 degrees Fahrenheit) water.
Get dust mite barrier covers for your pillows and mattress/box spring.
Keep any pets out of the bedroom.

If you think you may be allergic to the weed pollen outside (it is diminishing now, but still present), keep bedroom windows closed at night.

Consider trying an antihistamine such as Zyrtec or Claritin. You can get these without prescription at drug stores.

Then there is something called irritant rhinitis (or vasomotor rhinitis) and it can cause sinus/throat problems with mucus production too. It is from chemical scents, perfumes, tobacco smoke, etc. Consider using a scent free laundry detergent. And no laundry dryer sheets as those have toxic irritating chemicals in them that tenaciously adhere to clothes.

A decongestant can help for drainage, but if pseudoephedrine keeps you awake at night, you may not want to use it then.

Guaifenesin is a mucolytic (its the stuff that's in Mucinex) and is a safe medicine to take anytime. To get it to work well, drink 2 cups of water when taking it (or within a half hour or so of taking it) and it will help thin out the mucus so that it does not cause so much gagging.

Rinse your sinuses with saline before going to bed to help clear out allergens or irritants.

Now about that Flonase, some people benefit from doubling the dose of inhaled steroids (using twice as much, or 2x/day). If you find that it drips down the back of your throat excessively rather than staying put in your sinuses, ask your doctor about a dry nasal steroid such as Q-nasal. It is by prescription only and expensive, so you may not want it if your insurance won't cover it.

Don't do the sinus rinse within an hour of the nasal steroid as it will just make the steroid medicine drip down into your stomach.

I hope these things help. Please let me know if I can provide further information.

---