Sometimes flossing severely hurts, and I have battled gum recession for years. Could eating raw garlic regularly help kill bacteria in my mouth?

Flossing. If flossing hurts either your gums are inflamed (infected) or you're doing it wrong. You are probably in need of treatment by a bone/gum specialist. Please schedule with a Periodontist. Think of it this way, what you have been doing up until now has been ineffective. Time to see a specialist for care.
It might. But it probably will make little difference in your gum recession or periodontal issues. Flossing correctly should not hurt.