What's your phone?
If it's a CDMA you can call Koodo & have them "Clear your Switch" (they'll know what you mean).
If it's not a CDMA though, try powering down, removing the batter & SM, waiting 5minutes then put it back together & powering back on.
If restarting the phone gets the texts working again then odds are you are correct and the sim is failing. The text messaging centre number is stored on the sim and gets re-downloaded every time the phone connects to the network again. If it keeps hanging the sim is likely not retaining the number the way it should.