Hello Craig,
Sorry to hear you are having trouble with your voice.
I am a classically trained singer with allergies and asthma who sings sea chanteys and also teaches voice.
While it is possible you have done some damage, it is likely it can be reversed.
The earlier advice about throat clearing is good advice. Habitual throat clearing does irritate the vocal folds and increases production of phlegm.
For most people dairy products have no more affect on the voice than any other foods. Eating anything shortly before you sing will increase mucous production.
Having a lot of phlegm will not in and of itself cause a decrease in range. Other folks have made excellent suggestions about other underlying causes like GERD that can cause a change in the range and timbre of your voice. Drinking plenty of water is also great advice.
Singing is an athletic endeavor. If your throat is irritated and you sing less you will lose agility in the voice simply because you are singing less. You may also have changed your technique in an attempt to compensate for loss of range. Tipping the head back to reach high notes is a good example. Poor technique in any sport will lead to lessened ability and potential injury.
You may want to look for "Change your Voice, Change your Life" or one of Morton Cooper's other books at the library. He is a speech therapist who maintains that most vocal injuries or issues can be cured with out invasive treatments or surgeries.
If you are really interested in the physiology of the voice look for "Discover Your Voice" by Oren L. Brown, Voice Faculty Emeritus, The Juilliard School. He also favors voice exercises and training in proper technique.
It is also important to note that many medications and things like alcohol can irritate the cartilage, muscles, mucous membranes, and so on, that make up the vocal apparatus and cause a change in range or timbre of the voice.
Feel free to contact me offline if you have specific questions.
Sincerely,
Alison
alf@chantey.net