Welcome, GlennG.

Can you describe your usual sleeping setup? Tent, tarp, sleeping pad, clothing worn (or not)?

Sometimes a few adjustments can add many degrees to the bag you already have, and be more versatile in the bargain.

For example: I use a thin bag, but wear a fleece jacket and toque (stocking cap to you U.S. folks) to bed. That way the midnight foray to the john isn't quite so frosty. I have also spread a good space blanket over top, loosely so it doesn't trap too much moisture. Dry socks before bed can make a huge difference. And recently I'm putting a light fleece blanket underneath -- very cosy, useful around camp, and cost $5.

On the other hand, have you considered an overbag? Some of these are mummy-style, and you can stuff them inside your existing bag. Handy in damp conditions, because you can pull the two thin bags apart and they'll dry more readily.