If you plan ahead, like with an airline ticket, you can get TGV train trips for pretty cheap. When you get that cheap ticket, again like flying, you need to get on the exact train (and carriage and seat) that you reserved, and the reservation cost is absorbed in that cheap ticket.
If you either don't want to plan ahead or find that planning a couple of train trips is daunting you can consider a Eurailpass. Theoretically you just get the pass validated before you hop on the first train and go for it. The problem is that that is the way it used to be, and it still can be on the very slowest trains in France. You say you want the fast TGV trains. So you need to buy special Eurailpass reservations which are mandatory, and these cost a lot of money, and in France they are rationed so if the train you want has sold out of those reservations you can't get on even if there are empty seats. You only have two choices on the day - either wait for a different train on which there are still available passholder reservations, or pay the walk-up price for the seat. Plus you've also paid for that pass-day.
Or you can plan way ahead and buy the reservations as soon as you are sure which train you want so you are sure of being able to use that train. Of course all your flexibility goes down the drain. So paying for convenience now is paying for in-convenience.
Do the arithmetic and see which works out best for you.