I put too much motor oil in my car's motor. About a quart - have I ruined my car? I've been driving it. I know I need to get the oil drained. But I'm worried that I've ruined my car which has been running very well until now.
In short, no. If you read the answers on this similar question, even a gallon of extra oil generally won't ruin a car.
This is what I suggested and still recommend:
It's not that big a deal. With an extra quart, most normal cars driven by normal people will just have slightly lower mileage due to pumping losses. Go ahead and get it back to the correct level, though: no need to waste gas or trouble your peace of mind.
You most likely will not cause damage by running only an extra quart. Many people racing with stock oil pans will run half a quart to a quart of extra oil to prevent the oil pump pickup from drawing in air under racing conditions. Under sustained high RPM, oil can be pumped out of the sump faster than it can drain back. This issue combined with high cornering forces would move the remaining oil in the pan away from the oil pump pickup tube, causing oil starvation and engine damage.
Your engine's optimal maximum oil level is determined by filling up to where there is a safe gap between moving parts and the oil. What you put too much oil in your engine, you risk frothing the oil due to the crankshaft and connecting rods whipping air into it. The air bubbles in the oil will interfere with the oil's shear strength, which is what keeps metal parts from rubbing directly together. Not good.
Another problem is the additional oil splashing around the crankcase from the crankshaft and connecting rods whipping it up. It will coat the sides of the cylinder walls and could result in excessive oil consumption. In the worst case, carbon deposits from the burnt oil could cause the piston rings to stick, making oil consumption even worse and reducing power.
... but again, one quart is really not going to be enough for you to run into these problems. More than one quart is asking for trouble though. See: How far can I drive with 1 gallon too much oil?