When I go to bed at night I keep swallowing thick, white, stringy mucus that makes me cough and keeps me awake for hours spitting it out as it runs down my throat.

This could be from an infectious post nasal draining condition. Usually from an allergic reaction, so you basically have to think back and find out what you have been eating, what you have been near, such as cats, dogs, anything hairy and this would indicate a dander allergy.

In most cases drinking plenty of water will reduce this mucus, make it thinner, easier to deal with. Also, a non medicated nasal spray which is saline based and can be used many times because it is not medicated. I use hydraSense nasal decongestant.

What you are having is a normal reaction and allergies can happen any time in a lifetime, something that you were never allergic to before can crop up later in life, is what I mean.

This mucus is coming from your nasal cavities which are irritated by an allergy which is my guess, although we can't diagnose here on AMHD, so this is just my opinion. I advice you to make a visit to your doctor.

Tick

Meanwhile aside from brushing your teeth often, brush your tongue at least once a day and gargle with warm salt water as many times a day as you can.
Run your pillow through the washer and any blankets, coverlets too. If this happens only when you go to bed it might be a dust mite allergy. Use washable bedding and get at least one mattress pad under your sheets and keep them clean too.

My mom had this problem a lot. The best thing for you to do is to put a cup of water in the mocrowave for about 2 min. Then let it cool a little bit and drink it before you go to bed. It will relieve the coughing and will help the mucus not to build up. If it continues to slide down your throat you will have to go to the doctor and they will have to do a swab test on your throat to see where it is coming from. :)
Sorry to here about that, hope you get better.
Best of luck!