How do my fiancee(31F) and I(30M) fix and move on after lying?

I've been with my fiancee for a little over two years and overall everything has been great... except one thing. I caught her in a pretty big lie about multiple things. I 100% don't want to leave her and want to work on everything, but I just don't know what to do. She asks me what I need and what she can do, but I just don't even know.

Some background:

Over the winter I saw that a guy kept messaging her on Snapchat. I noticed because when we would scroll through social media together I would always see him at the top of her inbox, and I sometimes saw notifications pop up (disclaimer I always noticed that she left him on read though). I asked her about it and she said it was just some random guy from college that she barely knew, and he posts stories and sends them to people also.

A few weeks later I noticed he was still messaging her and I asked what was up again and told her I found it sketchy, especially being Snapchat. She said she understood, reiterated that it was some guy she barely knew and that she was never responding. I let it go again and then the next day she came home from work and told me she understood where I was coming from and deleted all guys off Snapchat that were not family or family friends.

About a month and a half ago I noticed again while we were scrolling social media together that the same guy now requested to follow her on Instagram. My alarm bells went off, and I again asked her who he is. I told her I find it weird that he gets deleted off Snapchat and then tries to follow you on Instagram, it seems like a lot of effort for a guy that you barely knew. I told her it seems like he's trying and wants to be in contact with her for some reason.

She again told me that he was some random guy that ran in her friend circle in college, they partied together occasionally and were friendly, but they were never really friends and just knew each other through other people. I felt like something was off and I pushed for more info, but she stuck to her story of a guy she barely knew in college.

I kept asking questions and she eventually broke down crying, she was upset that I don't trust her, she doesn't know how we can be in a relationship without trust, she doesn't know how we're going to continue without believing each other, etc. then she asked if she could have 5 minutes to collect herself.

She came back and told me that after thinking about their conversations she said there could have been some flirting, but it never meant anything to her, but she didn't know how he interpreted it from his side. I told her I felt like this was a trickle truth and half of the whole story and that more went on. She stood firm on what she said. I eventually let it go and we reconciled because I didn't want to ruin the week going into the long 4th of July weekend, but something still felt very off to me.

Yesterday morning while she was at work I decided to look on her laptop. Well, I found exactly what I thought. I found screenshots of them saying "I love you", they were using pet names, she sent him pictures (I didn't see the pictures, just the response), and the most damning evidence was that she was cheating on her boyfriend with this guy.

I felt like my world came crumbling down. Now I was worried that something went on during our relationship. I took pictures of the evidence and planned to confront her when she got home.

When she got home I told her "I will give you one more chance to tell me who this guy is, what's your history with him, and I'm so upset that I even need to ask this, but has anything happened during our relationship"? And I started crying in the middle of my sentence. She immediately started crying too and apologized and told me she'll tell me the full story.

Essentially she was in an emotionally and verbally abusive relationship in college (which is true, she's told me before, and some texts I saw confirmed it). She said she felt extremely alone in the relationship, she said she didn't have a lot of friends in college and felt alone, she felt awful about herself for being shy and not making new friends, she felt awful that the friends she did have always had guys interested in them, and felt like "a lonely fat loser that no one wanted". She said she created an online profile and matched with this guy and created a whole online persona to get him to like her and they talked for a long time. She said they'd go periods with and without talking. She said she never met him in person because he lives like 12 hours away (I looked him up and confirmed he does).

She said she has never told anyone about any of this, and she didn't want to tell me because it was such a dark and embarrassing time in her life and she didn't want me to judge her for doing something really weird. She said she thought it'd be better to make something up because it truly didn't mean anything and was no threat to the relationship, and thought it wouldn't come up and it'd be fine.

I asked her again if anything happened during our relationship. She promised and swore on her father who has passed that she never even responded to him.

I believe what she's saying but I'm still really hurt. I'm hurt that she lied about never cheating, I'm hurt that she lied about the people she's said "I love you" to, I'm hurt that she lied about who this guy was, I'm hurt that she thought it was appropriate to have him messaging her during our relationship and engagement, I'm hurt that she cried and manipulated me trying to make me believe her.

I told her I want couples counseling, I expect 100% honesty and transparency going forward, and this will be her one free pass on lying. I told her that I forgive her, I'm not mad at her, but I'm just upset. I told her I won't get over it immediately but I want to work on moving on, there will be times I get upset and she needs to understand that.

She understood and agreed to everything. She stayed home from work today because she's not feeling great obviously. She has asked me a couple times what I need or what she can do. But I just don't know how to answer that. I honestly don't know what I need in this situation or what she can do.

I want us to at least try to be normal and not have this hanging over our heads for days/weeks, but I honestly don't know what to do or what I need for everything to be better.