You're Overcooking Your Veggies
Keeping your veggies nice and crunchy through the cooking process both makes them tastier and keeps them more nutritionally dense. Start by just doing trial and error with your veggies. There is really no such thing as undercooking your common household vegetables.
Dry Your Veggies Before You Cook Them
Just like you should never eat a wet salad, you should always dry your veggies before you put them in a hot pan or oven. This is especially important if you're using veggies that soak up a lot of water like broccoli, brussels sprouts, mushrooms, and fragile leafy greens. If there is more water in the vegetable, you need to adjust your cooking time based on how you want your recipe to turn out.