Keyboard shortcuts for punctuation

I hate switching between keyboards when typing emails, and I use a lot of punctuation (comma, exclamation mark, parentheses, etc.). when I learned about the keyboard shorcut to get a period (click spacebar twice), I thought I could do the same for all my frequently-used punctuation. But I can't figure out how to get the spacing right.

For example, I used "cmm" for comma. In order to use that shortcut, I have to leave a space after the last word I type, them type "cmm" and then space. I end up with {word}{space}{comma} instead of {word}{comma}. Is there a character I can type into the shortcut line that tells ios to backspace before the comma? Or is there some other way to accomplish what I am trying to do?