Should I be her 2nd option if I believe her relationship won't work?

She knows I love her though I didn't tell her and is trying to keep me in her life while being with her boyfriend. They broke up and got back together like 3 times in the past. Once she even had a new boyfriend before they broke up and she came back to this guy. She never mentions her boyfriend with me. If I think they won't last long should I stay as her 2nd option?

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+1 y
She met her 2nd boyfriend and broke up with him before she met me
Should I be her 2nd option if I believe her relationship won't work?
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