Yeah, chicken breast isn't a great cut for a slow cooker. You want something with a bit more fat and a bit more connective tissue that won't dry out. Chicken legs & thighs are better.

We also really like the American test kitchen slow cooker cookbook.

I like using the slow cooker to get a hunk of meat cooked for the week. Tongue, chuck roast, pork shoulder are all great in the slow cooker.

I think chicken breasts are one of the worse things to cook in a slow cooker.

I would also look at the recipes you use. Read amazon reviews of the cookbooks you have, use websites that review the recipes like epicurious so you can get a sense of what works for other people.

Also, I really like America's test kitchen b/c they test often with recipes and equipment.

I think if you cook from frozen, the times listed seem to work out. Chicken won't dry out if it's covered with liquid.

I usually make a whole chicken on Monday and use it throughout the week to cook with. It's super easy -

1) Clean a whole chicken, remove the bag of parts if there is one (some do not have them)

2) Season with salt, pepper, garlic, cumin (really whatever I grab)

3) Put in the crock pot, cover with water.

4) Cook on low for 8 hours.

It is super tender and almost falling off of the bones.

So, we use a dutch oven for all of our slow cooker recipes and I have found that the time given in any recipe is always too long. Every recipe that I have that has called for 8 hours has never taken that long. Maybe it'd be different in an actual crock pot.

Until you get your confidence up, just cook when you are home and can check on the progress of things. If you are scared of having something be undercooked get a meat thermometer. But I have found the time quoted in recipes has been too long.

So, I'm a terrible cook. The last time I used the slow cooker my meat was like shoe leather.
I put some chicken breasts in the cooker on low. The recipe called for 5-6 hours. After 3 hours, I checked them, and the breasts were cooked through. Should I just put the cooker on warm?
Won't they be terribly dry if they cook for 3 more hours? Could the problem be that I doubled up on the liquid?