The price you see and the price you actually get are two different numbers, and closing that gap fixed my results
Early on I assumed the price on the chart was the price I'd trade at. It isn't, and the gap between the two is a real cost almost nobody accounts for.
Two things open it up. The spread, which is the difference between the buy and sell price and gets paid on every trade even under "zero commission", because that's where they've tucked the cost. And slippage, when your order fills worse than where you clicked, which shows up most in fast conditions and thin liquidity, so precisely when it stings. Your "entry at X" is really X plus spread plus a little slippage, and the same story on the way out.
On tight, high-frequency styles that gap can swallow most of your edge, and across currency pairs, indices or gold it stacks up fast over a month. I switched to fixed spreads on AvaTrade so at least the spread half is a known constant I can plan around, though the slippage half lives on every platform. The real fix is measuring your actual fills rather than assuming you got the chart price.
If you trade tight, go log the difference between your intended price and your real fills for a week. The number was a lot bigger than I wanted it to be.