I (26M) just found out my gf (25F) of 5 years cheated on me.

She had an infant son when we met, from a previous nasty relationship, and never held the expectation for me to become a parent. I thought to see the relationship through anyways because I was in a decent enough position myself and we got along very well. It turned into what felt like one of my happiest relationships I’ve ever had. I bonded so well with the kid and love them both very much. We had periods of ups and downs but we were always there to support each other through all of it.

In the latest slump, she became more cold and distant than usual. Lashing out at me over minor things that we’d typically talk through and became more possessive over her phone. She had just started a new job as an accountant in December and it was her first serious professional job. I figured she was just stressed out from all of it. She talked to me about all the folks at her work and she got along well with all of them. She was even telling me when she’d go hang out with them and I’ve gotten to meet a few of them. Nothing stood out. We had stopped having sex as frequently at the beginning of the year because neither of us had the time. She started coming home very late from some of her shifts beginning in February because of workplace demands. I took her at her word because she had never given me any reason to not trust her. Four nights ago, she fell asleep with her phone unlocked and my gut feeling led me to check it. Turns out she had been screwing one of her coworkers since February.

Needless to say that was one of the most heartbreaking moments I’ve ever felt. It was difficult to wrap my head around how someone I’ve learned to trust fully, who has integrated their lives with mine, who has made plans for the future with me, and who has allowed me and their child to develop such a close relationship could so easily betray me like that. I never expected this out of her. It bothered me that I spent most nights for the last two months being there for our kid and stepping up to taking care of more things around the house while she was screwing some other dude, because I thought she was busting her tail trying to establish herself at her new job.

I’m so lost and conflicted. She didn’t own up to cheating until I told her I read her messages. We talked for a really long time and had an honest conversation about everything, or at least I want to think it was an honest conversation. We both cried. She swears it was strictly physical with no emotional attachment. I sort of believe that because the texts didn’t reflect anything meaningful. Just plans to screw. She told me everything that happened, why it happened, and how much she regrets it. She told me I had become less responsive to her, but that was largely because of how much more she had been lashing out and being cold. I wanted to give her space because that’s what was communicated to me. She wants to work it out. She says she won’t block him for the sake of professionalism, but she offered to let me check her phone randomly moving forward.

I told her I needed time to think. In the meantime we’re still living together with 7 more months on a lease in both of our names, and two cars in both of our names that we have both split the costs of. We’re not married but I was planning to propose in a few weeks. I feel deeply hurt and used. I cannot understand how someone gets to the point of betraying a partner that loves them unconditionally and with everything going so well. It seems like she is being genuine about wanting to work this out and she’s taking steps to get therapy for us and for herself. But it seemed like she was being genuine when she reassured me nothing was going on and when she told me that she loved me, all while she was cheating.

I’m scared to think of what separating would entail. I have no chance of legal custody over the kid and I’m worried how much this will impact him. Our families get along well and we know each other’s friends. Our lease won’t run out for a while and it feels too expensive to separate right now. This is a huge boundary for me and I’ve made that clear from the very beginning. I don’t know how I can learn to trust someone again after this. I can’t stand the thought of losing her and the kid, but at the same time I’m so angry with her for upending everything and I want nothing to do with her. I think I just want to know if there’s even a chance of coming out the other end with both of us working out the relationship.

TLDR
I met my gf 5 years ago when she had an infant child from a previous terrible relationship and I’ve grown to love them both. Checked her phone four nights ago and I found out she was cheating on me with a coworker for the last 2 months. She swears it was strictly physical and nothing emotional. She has taken several steps to show she’s serious and willing to work this out. She doesn’t want to block him because of professionalism, but she says I can go through her phone any time. Our lives are deeply intertwined, but we’re not married. Separating seems incredibly difficult right now for many reasons (mental, financial, social) and I’m scared of letting go and that I’ll lose touch with the kid and how much it will affect him. Can we even work this out?