Yes, it's a decent chunk of my income.

Firstly, all bookmakers have their odds set so that if you back both sides, you will certainly lose

You probably copied and pasted most of this so I'm not sure who came up with that line, but anyone who doesn't find this obvious has no business in making decisions for themselves.

They basically set up their own arbitrage in every sporting event

You may think so but they don't. Often the aim isn't to balance the action, and even if it is they will still end up with a position. In reality they're making bets like the rest of us, just at much better odds.

Arbitrage is a hard way to make an easy living. You have to be organised, disciplined and have time on your hands. It's easy in theory, but in practice most people will have trouble.