The turn was your problem here.

You hit your straight, awesome!! Just what you wanted. So why are you flat calling (especially with the flush draw out there). Raise it up!

At least 3x his bet to go. The better players may tell you to push this I don't know. (I'm not one of them)

You raise big because you don't want to let him have a free card to suck out on you AND since you limped this pot for straight value and you hit you must get as much of your money in to make up for the times when you miss.

If he calls and he hits his flush thats poker. Possibly the reason you keep getting beat by flushes is that you keep letting villains have the odds to chase them.

Remember the golden rule: Bet your strong hands and don't give players pot odds to draw to hands that beat you. If you have the nuts there may be an argument for slow playing but only if you think you can get all his chips that way.