My 28M girlfriend 28F is upset I used to sleep with my taken friend 30F. How do I reassure her?

My gf (Ashley) says she feels betrayed by the fact she just found out I used to hook up with my taken friend (Sydney). She’s also questioning whether I been cheating on her throughout this relationship. (I have not and never cheated in any relationship). Now my friend and I used to hook up when I was single. Sydney has a fiancé (idk if she still does I haven’t been able to check on her since all her cheating came to light because things haven’t died down yet and my gf still doesn’t feel comfortable with me reaching out yet).

Recently her fiancé (Peter) found out she’s been cheating on him. He then went through her phone and found old Snapchat messages between us. Now that was before they were engaged but they were bf and gf. He then told Ashley. Ashley freaked out even though he even said he isn’t sure if we are still hooking up (we aren’t) because in old Snapchat messages I said “We gotta stop messing around soon I’m starting to get serious with this girl.😂” and she said “okay it was fun while it lasted.” I said “why yes it was.” Then the rest of our conversation was insignificant. The ones that had us doing something was way before Ashley and I got serious. Sydney doesn’t value loyalty in a relationship like I do, so she cheats but I don’t.

My gf doesn’t want me to talk to Sydney . She’s also questioning whether I like Sydney , (I don’t Sydney would cheat on me 💀). Then she’s becoming suspicious about any female friend I have. She gave me so much flexibility with both my male and female friends. Now she says we need to discuss boundaries when it comes to friends of the opposite sex even though I never cheated. & there’s proof in the text messages I stopped hooking up when things started getting serious.