First of all, get rid of the mirror that just causes him stress.
Second do a 50% water change with water which has been treated to remove chlorine etc..
Repeat the 50% water change daily.
Your tank is undergoing a cycle, basically as slt the levels of ammonia (fish excrement) are increasing greatly because there is nothing to convert them to harmless substances, your water now has a high level of ammonia and your betta is suffering as ammonia is highly toxic. The good news is that there are some friendly bacteria!! They should already be in your tank albeit in low concentrations - do you have a filter?
The bacteria approximately double their population every day so you can see it will take a while for them to build up, but starting with a few hundred in an average tank should be about 4 days before the ammonia starts to go down.
Then it gets a bit more tricky!! The ammonia which the nice bacteria converted to NitrIte is also toxic as the ammonia is converted levels of nitrIte in the tank increase! Fortunately there are another lot of bacteria which also should be in your tank (they are present in all but distilled or Reverse Osmosis water
) It will take them about a week to grow up into enough numbers to convert the nitrIte to nitrAte, for you and me nitrAte is just plant fertiliser. It is slightly toxic which is why you need to perform water changes of about 25% every week or so even when the tank is cycled (the bigger a tank you get the more stable so with a 3 gallon it'll need to be quite frequent ie every week).
Now by doing a 50% water change you'll dilute the ammonia in the tank by 50% it may prolong the cycle a bit as you'll chuck out some good bacteria in the water but it is definately worth it if you want your fish to live. When you petshop opens I suggest you buy "Ammonia" and "NitrIte" test kits it'll only come to £8 or $10 or so I would think.
Hope to help and keep us posted on your betta! And Good Luck!