Yes, I agree, you are doing a great job trying to get help for your little fishy. I know how you feel. Did you get a chance to take a look at some of the related articles here on the Fish Site at BellaOnline.com? There are some about fish disease and treatment that give practical advice. You may see something that looks like what your fishy has. I am so sorry that he is sick. How is he doing?
It sounds like it could be "Ich" if it is white spots. They can look cottony. There are Ich guard products at Petco and PetsMart. Aquarium salt as recommended in the previous reply to you here in the Fish Forum is also a good recommendation. It is an old-fashioned salt bath and is time-tested and used to treat various different conditions and less strong than medicines you buy in the petstore (less harmful).
Wow, you are such a wonderful person to be so kind and brave as to buy a sick betta and nurse him back to health and keep him healthy that long! Otherwise, he would have died in the pet store. That always seems so needless when I see that. Thank you so much for saving him and loving him.
Another Betta disease is called Bloat and to cure this it is recommended that you can treat it with tetracyclin, very, very low water levels, and a plant for balance.
How cold exactly was your house and for how long before you warmed up your fishy with the light? What is his water temperature now? Do you have an aquarium thermometer on or in his fishtank?
What size of tank do you have him in? You can get some Betta tanks at PetsMart and Petco or other petstores and aquarium stores that have lights in the hood and you can also buy aquarium heaters of all sizes - even an aquarium heater made specifically for a small tank. Yes, it can get way too cold for Betta fish to be left at room temperature in most of the world in winter time. They are tropical fish and can develop various illnesses in cold water.
How is he doing? There are a lot of fish-loving people viewing your post and wondering the same thing! We know what it's like to feel that worry and sadness and we want your little guy to feel better - and you too!!!