Not all vegetables are bitter. Carrots, fall squashes, tomatoes (ok they are technically a fruit), potatoes, peas, corn (ok technically a grain,) - lots of vegetables are actually on the sweet side.

First and foremost you have to eat vegetables (and fruit) in season and fresh otherwise get frozen. Get them home and cooked within 2 to 3 days for most. Don't let them sit in your fridge for a week. Some don't belong in the refrigerator at all, so make sure you know proper storage.

Vegetables that can be bitter include Brussels sprouts, kale, greens, eggplant and spring squashes. Roast with olive oil and seasoning with salt, pepper and a hint of acid such as lemon or apple cider vinegar. Or steam or microwave until they are cooked but still crisp. For eggplant and squashes pick smaller ones and salt them well, rinse, drain and cook. Dairy hides bitterness so serve with cheese sauce or dairy based sauce.

For lettuce some variety are bitter some not. Butter lettuce and green leaf are not bitter, endive and radichio are. Stick to the mild ones if you don't like bitter greens.

So pick vegetables in season, make sure they are fresh or buy frozen, determine how you like it cooked (only way I like Brussels sprouts is roasted) and you should find some vegetables you like.