You need to meet with a dietician (NOT a nutritionist, there’s no certification for those at least in the US) to discuss your nutritional needs.
What I recommend as a starting point to people who have no idea what constitutes proper eating is to have 1/2 their plate with veggies (corn, beans, and potatoes are not veggies), 1/4 lean protein, 1/4 fibrous starch so like corn, brown rice or other whole grains like barley, quinoa, legumes like lentils or beans, potatoes/sweet potatoes, whole wheat bread, etc. Fats should be used sparingly and for the sake of just getting started it doesn’t matter if they come from plants or animals as the point right now is just to build a habit. Added sugars should be avoided; sugars naturally occurring in fruits and so on are fine.
Once that’s a habit, learn about portion sizes. 1 serving of meat is about the size of a deck of cards. Yes, that small. You don’t need to go back for seconds, really you don’t. If portion control is an issue for you, make your plate and put away the leftovers before eating. If at a restaurant, order something that can reheat well and ask for a to-go box to be delivered at the same time as your meal so you can put the rest of it away.
Food may taste bland to you at first because you’re probably used to a lot of sugar, fat, and salt. Find some spice blends and experiment with them instead. I’m a big fan of Mexican, Indian, and Asian cuisines for tons of flavor that doesn’t have to come from fat and sugar. Add salt judiciously. If you need to rely on sugar-free treats for a while to ease your sweet tooth, that’s fine too as you’re making a lifestyle change, not a diet. Over time your palate will adjust. And drink water or herbal tea regularly. The 8 glasses of water thing is a myth, but if your pee is darker than a medium yellow then you need to drink more, most likely. It’ll keep you full longer, will help with your skin tone and general appearance, and proper regulation of your digestive system among many other benefits.
For MOST people, dietary cholesterol doesn’t affect blood cholesterol. If you eat the plate portions I mentioned above, it’ll likely go down regardless of what you eat.
Again, remember that this is a lifestyle change so if you need to make one small change at a time to avoid giving up altogether because you can’t maintain perfection, then that’s fine.
Try unflavored whey so you can use it in anything and use actual spices and other flavoring agents to the oatmeal to generate flavor. For example, if you want vanilla, add some vanilla extract. Chocolate, use unsweetened cocoa powder. For oats with nuts and berries, I use my own pie spice mix that that has cinnamon, cloves, nutmeg, cardamom, and ginger.