'open this when you need a reminder of how much i love you'
Exact translation is a little difficult. I would say it like this (I think it's close enough).
- 내가 얼마나 자기를 사랑하는지 확인하고 싶을 때 이걸 열어 봐요.
* 내가 얼마나 자기를 사랑하는지 = how much I love you.
* 확인하고 싶을 때 = when you want to confirm it to yourself. (there seems no common word for "reminder")
* 열어 봐요 = open it.