Should I tell her what happened?

Okay, to cut a long story short, i recently broke up with my girlfriend (3 weeks ago).

Throughout our relationship she was worried that I liked another girl who I work with (wanted to sleep with her haha but not in love with that girl) but anyways, after we broke up I have been seeing this girl that I worked with and we have done some sexual things (but not gone all the way) We are not seeing each other anymore though (met about 3times)
I realised my mistake and i asked my ex girlfriend if she wanted to meet and talk in hope of getting back together.
I know that she will ask if I have been seeing my coworker, but should i tell her what happened between us? won't that just destroy any little hope that i have left of getting back with her?
Should I tell her what happened?
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