You made a lot of good decisions in regards to your fishkeeping, but unfortunately some poor information has tainted your experience. I wouldn't let it dissuade you from trying again, though.
First of all, most pet store employees don't really know what they're talking about. A heater is definitely required for keeping a healthy betta. After all, they are tropical and thrive most in temperatures from 74ºF - 80ºF degrees. Not only does a heater keep the temperature warm, but keeps it stable - also necessary for keeping your betta healthy. Temperature fluctuations can stress your fish and weaken their immune system.
Secondly, you shouldn't have to purchase your water. You can use water from your faucet and dechlorinate it with any conditioner. If your water is high in ammonia, you can buy Seachem Prime conditioner which will take care of that issue. If it contains metals, you might need to filter it. You should also be doing two water changes a week with a tank of that size. I'm not sure, but you might be able to pull of one 50%, and one 100%, instead of two 100%.
Third, it's good that you've purchased several types of food, but freeze dried blood worms should only be fed as a treat. Probably once a week, only. They have little nutritional value compared to the flakes or pellets and don't contribute much to their diet. Feeding a few pellets or flakes once or twice a day is normal, as is fasting one day of the week to prevent any constipation or bloating.
Lastly, your tank looks great and your next fish should live happily with all adjustments considered. I'd remove that plant, though, as it's not truly aquatic and find a pet store that has tanks of live plants kept underwater, rather than in tubes. Rotting leaves will only contribute to rising ammonia levels. Good luck with your next fish, and keep us posted on your progress.