To cheer up your girlfriend, you can try the following:
1. Apologize sincerely for whatever caused her to be upset.
2. Listen to her feelings and validate them.
3. Offer a gesture of kindness, such as bringing her favorite snack or giving her a hug.
4. Plan a fun activity or date night to spend quality time together.
5. Write her a heartfelt letter or message expressing your love and appreciation for her.
Remember, communication and showing genuine care and affection are key to resolving conflicts and making your girlfriend feel better.