Lisa, do not stress. Really. Just be grateful that you will not have to drive on that other, funky side of the road they drive throughout the UK. :)
So, if you rented a car with an automatic transmission, chances are good that you will get one. We picked up rental cars at Avignon TGV, at Aix TGV, at a downtown location in Nice (neither airport, nor train station), at a Toulouse train station, etc., and did not yet have a problem getting the automatic car that we reserved. Of course, now having opened my big mouth, there will not be an automatic waiting for us in downtown location in Nice this June, but we will deal with it then. :)
If you rented your car through AutoEurope, and if the automatic is not available by some chance, you could call their 24/7 line from France and I am sure they could assist in some way.. perhaps you could swap cars at the next hub.
About driving.. honestly, it is no biggie in the countryside (not any different then in a small town USA) and if you make a mistake and take a wrong turn, you can always stop at a shop or a gas station, and eight times out of ten there will be someone who speaks English (assuming you are not fluent in French, but I could be wrong here), or who is willing to take the time to do some pantomime and help you get on the right track. Been there - done that :) before the GPS.
Now, I have all the maps with me in the car and kind of follow our journey along as husband drives. However, we rely 90%+ on the GPS for our directions. Every once in a while, the GPS will take us down a narrow, strange road, but we can always "override" it by not taking the narrow, strange road, and it will re-calculate a different route.
The issue is that we bring our own portable GPS with French maps downloaded on it, so we are very familiar with how to operate it. It also instructs in English. I am sure that the maps will be current on a rental GPS but you may want to make sure that the verbal navigation instructions are in English before you leave the rental office. You may also want to practice a bit punching instructions into it before you tear out of the rental office's driveway.
Essentially, you will spell the city/town/village you are going to and if you have the street name, great. If not, you will merely pick from the provided list of possibilities the road number you want to travel to get to that place. This is when your Michelin map will come handy, so that you can see which approach you would rather take.
The GPS works great for us and at least during the last five years, saved us a lot of heartache while traveling in Europe. I sure hope that you will find it helpful also.
Stay positive - you will do fine!