To decrease the bitterness, bring out the sweetness in the carrots by cooking for longer. The best technique I've found to do this with carrots is frying and steaming them at a low temperature.
Make glazed carrots: per 200g of carrot, peel, cut into ~2cm thick diagonal cuts and add 40g butter, a half teaspoon of sugar and season with salt and black pepper. In a large lidded pan over a low heat, spread the carrots out in a single layer, add the lid and cook for 25-35 minutes. Shake the pan occasionally. Push the cooking time as long as you can without letting the carrots become overly soft. This should give deliciously sweet fragrant carrot.
Another thing to keep in mind is that salt is better than sugar to reduce bitterness so add as much as you can without oversalting the carrots.