I had negative non showdown winnings until recently. I have been a break even player until recently.
A few things I was doing wrong.
You give up too early on certain players (calling stations). For example, you CB a flop with top pair out of position, get called, then check the turn. You just told the villain that you either have nothing, and slowed down when he called, or that you're not married to the hand. He bets turn and takes it down. A lot of players will call the flop bet, and hope they hit the draw.
You play too many hands out of position.
You don't use position to your advantage as much as you could.
You fold when you are ahead too often. Just because a guy shows strength, you shouldn't assume that you are beat. Does his line make sense? Does it make sense when you consider his actions on all streets?
Call/Call/Fold Syndrome. Have a plan.
Play draws in certain situations aggressively. If you only make money when you hit with draws, you probably aren't making as much as you could. For example, Kh9h2d flop and you hold Ah2h. You are probably ahead at this point. If you raise, you have fold equity. If you check/call you don't. Raising also builds the pot for when the flush does hit, and it can also disguise your hand.
There's more that's not popping into my head, and I'm sure there's more that I haven't figured out yet.