Hi! I found a similar question asked elsewhere, so the answer there might help you:

If you truly believe he is best for her and as her best friend, you would give her boyfriend your stamp of approval, then you'll need to come clean with her.It's the respectful and right thing to do. . .Because the longer you are supporting her as her best friend, without telling her, you are essentially lying to her too.So tell her you want to continue being friends with her and her boyfriend but the truth is you have recently recognised you have developed feelings for her and you know you need to be away from her for a while in order to sort of wipe the slate clean and then come back only after you're in a better place and in your heart you see her more as a sister than a girlfriend.Only you can determine how long it will take but be truthful with yourself and while your life is seperated from hers perhaps you’ll find someone else that can offer what she has.

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