Q: Car cannot shift gears while idling in traffic

I’m assuming you are talking about standard transmission. If that’s the case, either your clutch slave cylinder or master cylinder is going out.

Check to see if it is leaking any fluid and check if it feels mushy. If it’s worn out, you’ll need to replace it. These are cheap things to replace on any vehicle.

If it’s not that, then the clutch itself is wearing out and you’re going to need a new clutch. A clutch job is pretty expensive and most people aren’t going to it themselves. It’s a big job, if you need assistance with this have a certified mechanic diagnose your shifting issue firsthand. Afterwards they will able to repair it accordingly.