Making someone happy can be a difficult task, especially if they are feeling down. The best way to make your girlfriend happy is to show her that you care about her and that you are willing to listen to her. Spend quality time with her, ask her about her day and her feelings, and let her know that you are there for her. Additionally, you can do something special for her, like making her favorite meal or taking her out for a fun activity. Above all, be patient, understanding, and supportive.