should it bother me that my best friend is dating my ex boyfriend?

Me and my ex-boyfriend were together for about a year, around six months ago I had ended it purely to the fact he had cheated on me and lied several times about it. My best-friend was there while he lied to my face and she seen how emotionally upset i was due to this incident. I don't want to do into detail about how he had cheated on me, i guarantee you can figure that out yourselves. Any way, about a month ago I bumped into her and questioned her about the so called rumor, she denied it and told me it was her close friend who was meeting up with him. Obviously I thought I could trust her and took her word for her and left it as that. Earlier today he messaged me telling me he loved me still, ect.. And i felt the need to question him on the matter, he admitted everything about them two dating. Do I even have a right to feel anger towards the pair of them? Should i forgive her for lying to me? I'm just so confused on the situation. oh p.s she's done the exact same thing to two of her "so called" best friends, I just didn't see her doing it to me considering I've been so close to her since I was about four years old. Any advice would be truly thankful!x

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