Try adjusting how much exercise you do -- or the time of day when you do it . . . We quit because we get to an "I don't feel like it day", or we are seriously burnt out. You either do less on days like that, or you push yourself past the "I don't feel like it" mood.
I used to swim a mile every day after work before going home. (I was single then) Some nights, I didn't want to bother, but I drove to the Y anyway. I would get into the pool thinking I wasn't going to push myself, and usually, I think because I started slowly and with no expectations, I would generally swim my best lap times.
Find a time to exercise and an amount of exercise that will allow you to be consistent about DOING it. After child #4, I decided I would run in the mornings, before I showered for work. This often meant running in the dark, and that was kind of scary for me, but I did it . . . later, when I shifted to running after supper, I was AMAZED at how much more energy I had, having been fed before I exercised. If you run, walk or bike, be sure you leave a written note at home with your route listed.
And other than that, when you "burn out", instead of stopping exercising, simply cut back a little. At least do SOME, so you feel you are continuing with the game plan. And you may find that once you start, you just keep going. Starting, for me, isn't half the battle -- it's the WHOLE battle.