How should I end my relationship?

I been dating my ex from the year of 2023 since September. I really like her. But she has her flaws though but we have a lot of fun together and we are just like yeah were together. BUT a day before Labor Day. My other ex from last year came back and we are working towards a relationship again. This coming Sunday she wants to make it official since it's like our 6th date. Now this 1 I have so much in common with and she shows she cares and I feel like a spark. I have decided who I wanna be with. But how to do it any advice?

How should I end my relationship?
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