I always feel incredibly full and nauseous after I eat anything, literally. This has been happening for the past two years. What's wrong with me?!

Heartburn? Most common issues would be gastritis, ulcers and chronic constipation. A little less likely, celiac disease - intolerance to gluten (wheat protein) depending on your age, celiac stenosis (blockage of an artery that feeds the stomach). Less likely, pancreatitis. Even less likely is a bezoar - ball of indigestible material accumulated in the stomach. (do you chew on your hair?).
Satiety & nausea. The perceptions of satiety (fullness after eating) & nausea are complex processes that involve the brain, stomach, hormones, & the composition of food. A persistent problem raises concern for gastroparesis (delayed stomach emptying) due to neurologic &/or structural problems of stomach & surrounding organs, small bowel problems, & even an issue involving the emesis center of the brain. Get checked.