First of all, if you find that your girlfriend has cut her double eyelids, you should respect her choices and decisions. Everyone has the right to decide his appearance and image. Cutting double eyelids is also a common plastic surgery, which doesn't mean that she is untrue or doesn't love you.
Secondly, you should consider your own feelings and needs. If you think that the matter of double eyelid cutting is very important to your's relationship, then you should communicate with her frankly and understand her thoughts and feelings. If you think this will affect your's feelings or relationship, then you can discuss ways to solve the problem together.
Finally, no matter what decision you make, you should consider each other's feelings and interests. If you decide to stay with your girlfriend, you should support her choice and decision, and don't let it affect your's feelings and future. If you decide to break up, then you should end the relationship in a mature and respectful way, and don't hurt each other's feelings and self-esteem.
In a word, it's no big deal to find that my girlfriend has cut her double eyelids. The key is communication and mutual understanding between you. If you can deal with this issue in an open and respectful way, your's relations will be more stable and lasting.