Toss out the cos; dump the iceberg while you're at it, too. Lettuce is crap. Lettuce has always been crap. And here is why.
It's Mostly Water
Let's start with nutrition. Lettuce has approximately nothing in it. You figure it must be healthy because there's so much of it in your salad bowl, and it's a vegetable, right? You're eating so many vegetables! Run the numbers, though: A serving of cos is 85g, 80 of which is water.
In the remainder, you get a measly 2g of fibre (we need 25 or more each day).
Surely it's got vitamins, though? Yes, but not much compared to what's on the rest of your salad. Cos does have your full daily value of vitamins A and K, but compare: that means it has as much vitamin A as a quarter-cup of shredded carrots, and as much vitamin K as a few sprigs of parsley. Both of which, I might note, have more flavour.
Iceberg is even more useless: a serving has just one gram of fibre, and nearly all of its vitamins in single-digit percentages of your daily value. Just chuck it in the garbage. You'll never miss it.
Oh, and if you're happy that it's "mostly water" because you need to hydrate? First of all, no you don't. And second, that whole bowl of lettuce gives you 60mL of water. Just take an extra sip from your glass instead.
Salads Are Fine Without It
So let's try a little experiment. If you take the lettuce out of your salad, what's left? If it's a hearty, healthy salad, you'll have plenty of veggies, legumes, grains, and other foodstuffs both delicious and nutritious. If it's a Caesar salad, you just have cheese, breadcrumbs, and a puddle of dressing. If it's a side salad, you might only have a few drops of vinaigrette and a single decorative cherry tomato.
This is why you're always hungry after you have a salad for lunch: you barely ate anything. You just didn't notice, because you spent half an hour chewing on so much useless mostly-water. Try making your salads without lettuce, and learn what it's like to eat real vegetables.