I just completed my open water certification and I realized I have one more question I forgot to ask: How do you control your ascent speed if you've lost your mask and can't see your depth gage? Of course I'm assuming my dive buddy can't help for some hypothetical reason.

you can use a method I used years ago before computers, go up no faster than yout smallest bubbles. try it it works.
divedude

The best way to avoid this is to stay close to your buddy. If you do get separated and need to ascend without your mask, you should be able to see at least a fuzzy version of your gauges. If you have a computer, most of them will beep or at least flash at you if you are exceeding their recommended ascent rates. This would give you an audible and/or fuzzy flashing clue as to sow down or not. Also, EMPTY all of the air out of your BC. If you're not overweighted, a gentle even kick will get you to the surface. Watch your bubbles and rotate while you ascend. Salt water is not harmfull to the naked eye and you will be surprised at the detail you can actually discern with your eyes open.