First, make sure you’re measuring your ingredients correctly. Fruit can sometimes be difficult to measure, especially if you’re working with large pieces. Second, I suggest using fruit that’s as ripe as possible. For bananas, that usually means once they’re browning. Third, consider using milder leafy greens. Spinach is usually on the sweeter side, but if you don’t like the taste try substituting with romaine lettuce instead. Finally, if you still find your smoothie on the bitter side you can adjust the fruit and veggie ratios so there’s a little more fruit and fewer greens.