Struggling with position sizing and the 'cost of waiting'

Hey everyone, been lurking for a bit and decided to finally jump in. I've been paper trading for about six months and starting to feel more comfortable with my strategy (mostly scalping $EURUSD and $GBPUSD on the 5-min chart, with some core swing ideas on indices). The biggest mental block I'm hitting now that I'm considering live trading is around position sizing, specifically when a setup takes a while to confirm or price action just chops around my entry for hours.

My backtesting shows good results when I enter at my pre-defined levels and the move happens relatively quickly. But in real-time, there are so many instances where I'm sitting in a trade, maybe slightly profitable or slightly down, waiting for confirmation, and that capital could be deployed elsewhere. I know the standard advice is to stick to the plan, but how do you experienced traders balance the 'opportunity cost' of capital tied up in a slow-grinding trade versus the risk of exiting too early and missing the eventual move? Is it just a matter of tightening stops or accepting that not every trade is a quick hit?