On that note, know that I am not yet profitable like lots of the ol' boys around here. But I am, with my most recent level-headed trading, breaking-even. I feel that is a pretty good milestone in itself.
I'm going to go against the grain and suggest taking a look at what goes on in-play. Most of the trading advice is for pre-race markets. Personally I never found that interesting nor very profitable. You are competing against every other bastard doing the same thing, so you're playing a kind of meta game. You learn a lot about the markets, but not so much about what drives the markets.
What I love and simultaneously hate about in-play is that it forces you to look at what is actually going on in the real world- the actual events driving the prices. Just looking at pre-race it's easy to forget the fact that the prices are merely a representation of the probability of events occurring. The real world events drive the prices, not the other way around.
When you start wondering why this or that trade went against you, it's very easy to start blaming the market. But it's hard to argue that if the price goes against you because your team or horse lost, that it's the market's fault.
Anyway, just my 2 cents worth. I've lost more money on sports than I've made on horses. But the latter makes up for my mistakes on the former and I only trade in-play. Come to think of it, if just stuck to horses I'd be lifetime profitable by now
Managing expectations is a big one, the frustration that OP talks about is understandable, trading looked so easy from the outside that I was convinced I would master it within 2 weeks maximum. 
We all have to do our bit to promote the exchange 
