I (28M) found out my gf (26F) was seeing someone else during the beginning of our relationship

I (28M) have been going out with my gf (26F) for about a year. We started going out July 3rd last year. We talked about exclusivity within 2 weeks of that day and she said she wasn’t seeing anyone else and I said the same.

Recently I found out on July 14th she was seeing someone else. I found this out when she was showing me her previous reservations and I saw one to a high end restaurant and it was for 2 ppl. I asked her about it and she said no she was gone with friends. I said it was only for 2 and she said they called and made it for 3. I asked to see pictures because she loves taking pics every where but turns out she had none. The next day she decides to come clean and she was seeing someone else during the time.

I asked her for more details and she said she went out with this person 3 times while we were going out. July 14th was the last day she went out with him and she only went there to end things with him. I went back and looked at our texts for that day and I asked her to hang out but she said she’s going out with her friends and it turns out she was lying.

I asked her why she lied and she said I assumed I was doing the same thing. I said why would I lie and she said can u blame me I have trust issues from the past. I asked her why she didn’t come clean and she said she didn’t wanna hurt my feelings and it was just a little white lie.

The way she’s talking about makes it seem like it’s not that big of a deal am I going crazy cuz I think it is? And other advice would be greatly appreciated.