Hi Jennie
I also used to reward myself for doing what I thought was a fairly boring, mundane task. Unfortunately, being a stay at home mom, there were plenty of those.
I found telling myself that the 'reward' system was absolutely ridiculous. Instead of seeing booze as a reward, try picturing it as a poison. "After mowing this enormous patch of grass, I'm going to poison myself." Doesn't sound so brilliant, does it? Reward your body and mind with a huge bottle of ice cold soda or water. It will hydrate your system so much better than anything with alcohol in it, and in that hot weather, it's what you should be doing, anyhow!
There are many, many triggers ahead of you, this one seems a biggie, but in the grand scheme of things it's tiny - and you'll see that tomorrow. You can do it!