First, I assume the water damage is from water splashing up on the mirror, getting behind it and eating away at the mirror surface on the back of the mirror…right? If there’s something else going on, you should deal with that first.
Second, I’m not that handy, but trim is cheap and I don’t see why you couldn’t take a shot at it yourself. Get the some PVC trim, cut it at 45s and, IMO, glue it onto the mirror itself. This’ll be a lot easier than mounting it on the wall and trying to overlap it onto the mirror. You’ll have to chisel/route out the area where the clips are, but it’s doable. Dry fit it first since you’ll only have one chance with glue.
Another other option is, as was mentioned, two (or more) smaller mirrors, picked up from Home Depot/Lowes. But if your sink is in the middle, that could be awkward.
The third option is to call a glass shop and get a quote for a new one. If it’s a big mirror (and you said it was), I’d ask how much it is to have them deliver and install it. I can’t imagine it would be that much. But even a hundred or so would be, IMO, more than worth you trying to get it home and wrangle it into place. Let the pros do it and let them be responsible if it gets broken in the process. They’ll probably even take the old one with them when they leave.
Here’s another option I just thought of. I too am awful at wood working, really, just terrible. You could get a nice matching piece of frame and just run it across the bottom. If it’s a neutral color (like white PVC), it probably won’t be that noticeable) but will hide the stains. As (some) people say, you don’t notice clean, you do notice dirty. People probably won’t notice random piece of trim on the bottom of the mirror, but they will notice the water damage. Doing it this way will require only one straight cut and some careful gluing. That’s miles easier than 8 miter cuts, lots of measure, lots of lining up and lots of wondering if you should have called a carpenter.