My horse was diagnosed two weeks ago and started on oral dex. We did:

10mL x 3 days

7.5mL x 3 days

5mL x 5 days 

2.5mL 5 days

Initially, there was a slight improvement in my horse's breathing during exercise. Today was his last day of  2.5mL and his breathing was just as bad as ever after a light ride. My vet put him back on 10mL abs said we'll taper him more slowly. 

He lives outside 24/7 and has not had hay in a year. I just bought a haygain steamer for him but haven't used it yet. 

Any experiences that may help me??

Mine was just diagnosed as wel! But mine is triggered by dust and mold spores. My vet made me a special blend of DMSO, DEX, saline, and Gentimycin that I use a few cc 2x daily in a nebulizer. Works like a charm!!! Nebulizing it is so much safer than orally or IV systemically. When you nebulize you target the specific tissue that needs it. I also dunk his hay in water and do have him on curost total support. He feels AMAZINg !!

Boringher  Ingelheim came out with an equine inhaler treatment called Aservo, especially designed to treat horses with COPD. It's a ten day treatment but that one treatment period is supposed to last a few months. 
you can search YouTube for a video on it. 

I have one and hes IR on top of that, so got to really pay attention on what I try to help him with. 

Bump

I asked my Vet about it and she said its not good for long term chronic issues. Only short term. She had not heard about it lasting that long. I watched the video and it looks really cool!