This is a common enough problem, Michael. It often takes people a while to become accustomed to mummy bags—they're just too confining, and they really aren’t that fun to sleep in.
There are reasons for that, of course. A slim cut saves weight and reduces the amount of cold air the body must warm. But both of those concerns are mitigated somewhat when you go car camping.
The opposite of a mummy bag is a rectangular sleeping bag. They’re shaped like a letter envelope and allow all sorts of room to move around. They're also heavy, but when you’re car camping—or even canoe camping —who cares?
Prices for rectangular bags are all over the map. The Kelty Callisto is a synthetic-fill bag that is rated to 15 degrees and sells for $90. But the Big Agnes Summit Park has a similar temperature rating and sells for $269. The difference? The Big Agnes bag has down fill, which is much softer, more comfortable, and keeps its loft and warmth for years longer than synthetic.
As an intermediate step, you might look into a semi-rectangular bag. These bags are wider than mummies but retain some taper for the thermal and weight benefits. REI’s Siesta +20 is a good-quality synthetic-fill bag in this category. Western Mountaineering’s outstanding Alder MF is a down-filled bag rated to 25. It's beautifully made and sells for $340.
Your best option, though, could be Mountain Hardwear’s synthetic-fill Megalamina 20: it's a double wide, meaning it fits two people.