Basically, bookies adjust the line, that is, the payouts, based on a combination of knowledge of the sport involved, i.e. their general take on who is more likely to win and how much more likely they are to win, and often more importantly, the action from the betters. Essentially, the bookie wants to set a line such that no matter which outcome happens, the house wins.
The result is there are sometimes lines that are skewed by public opinion to the point that it's positive expected value to bet on the opposite of the popular result. There are also arbitrage opportunities where two different betting sites are offering odds such that you can bet both sides and eke out a small profit no matter what happens, much as the bookies do.
Where gamblers end up getting into trouble is doing some version of the Martingale strategy, which is quite simply doubling up your bet on a nearly even money (but still losing) bet every time they lose, so that the majority of the time, they get a small profit, but when they do lose, it's huge.
In any event, betting big is generally a bad strategy at least unless you've essentially marked off part of your money as shit you can lose without caring. If you want to gamble effectively and maintain it, without impacting your life adversely, the gambling budget is entirely separate from your other money, and your largest bet is based on what this specific bankroll is likely to survive over the long term.
And seek out opportunities where you have a slight edge (good luck finding a big one), because in the long run, with anything where you are even at a slight disadvantage, there is no bankroll big enough. Look up Kelly criterion for the general concept of "risk of ruin" and how to maximize a small advantage.
Sports betting is sort of the graveyard of the degenerate gambler. Stu Ungar, one of the greatest poker players of all time, won millions at poker and lost millions because he was absolutely terrible at sports betting. Almost nobody is good enough at sports betting to beat the bookies.
