Like you and many others I may have eaten horse meat without knowing it. I've certainly eaten it deliberately in the past.
It doesn't worry me that I may have eaten horse meat instead of beef, but the potential you've mentioned - of contaminated meat finding its way into processed products - does concern me. That said, I don't know how anyone could devise a foolproof method of preventing it. It's easy to say that our government should introduce monitoring, but in truth it would be almost impossible to produce a system that worked efficiently.