I don't want people to gang up on you either. I just want to confirm for you that the lack of a heater is the issue.
If there is a warmer spot in the house where you can keep the fish to at least get to 76 degrees try that. Personally I keep my betta at 80 degrees. I put 76-82 degrees as a range for them with 78-80 being my ideal. Between 70-73 the fish will suffer from lethargy, loss of appetite, and disease.
One possibility for you is to put the betta in container and put it in your 55 gallon tank until you can get a heater for the 10 gallon. That way it won't interact with the other fish but will get the benefit of the tropical water.
As a side note be sure to do water changes regularly as the uneaten food will create dirty water.