Should I leave for something i found out over a year ago?

Me and my partner have 2 kids together, he also has one with his ex, ever since we've lived together she's always made excuses and being controlling its very much has to be her way or no way, so she gets picked up at 10 then we have to have her back for when she says, hes been wanting her overnight for over 3 years now and she changes her excuse for why she can't stay, initially it was the fact that we stayed at my parents house( understandable) then she said her room wasn't done up to the standard that she wanted ie. (We didn't have the right set of light, or my personal favourite... the bed is too small for her shell need a bigger one ) the bed that she asked us if we wanted for her room), and after that she said hes too inconsistent...

My partner works night shift she knows this and therfore struggles to get up really up, I've suggested what if we had her afternoons, she said no has to be ten (this is a conversation they had). I thought I'd be nice so I reached out her basically saying I've no issue with picking her up every once in a while when dad's struggling to get up, to which she said "no has to be dad, his responsibility, one of the reasons we broke up" (his responsibility even tho hes asked to change the time so its easier for him but you wont allow it)

Anyway I got curious and asked why they broke up just to see what she says as he says its because he was never able to see the child as she was always at her granny's when he wasn't sleeping for work and he asked her on multiple occasions to cut it down or go when hes at work instead of home. She gives a few reasons blaming him on the breakup, before she tells me that they where still together until he moved into mine, claiming they where doing stuff together, I confronted him, he admitted he lied about where he was staying as he claimed he stayed at his mums, but denies doing anything with her from long before we were together, claims he slept on the sofa, whereas she says otherwise. One of her words to me was "im telling you because if I was in the same position I'd like to know" to which my partner says "I've been to her house for drop offs and pick ups she could've gotten any of her family to message me if its a case of I'd like to know aswell but she didn't she waited for you to message her being nice" this was 2 years into our relationship. And a very fair point he made,, he was at mine every single day before moving in he practically lived at mine already only left at certain times when he had to take the child, he was sleeping in my bed so how was he sleeping in hers at the same time? If she knew I was pregnant before he told her and she means that she "wouldve liked to know" then why wait till now to tell me?

She also sent me 2 screen recordings of their messages while we were together sending each other xx (he claims it didn't mean anything and was just a habbit) and another not so appropriate message that he says was just a joke as its not something she ever done and was said in a jokey way.

Since then its been about a year, but i cannot stop thinking if anything did happen while he lived there, im not concerned about the current time I can whole heartedly say he currently isn't cheating on me as we spend all day every day together unless hes working and even then its me dropping him off at work, but i can't stop thinking if something did happen while we were together even if it was at the start, I simply don't know who to believe, should I just try put it past me as it was in the past almost 3 years ago, or is it bad that it still causes me to overthink simply at the fact that something could've happened and I simply don't know,,, its getting to the point im overthinking it so much I've been wanting to leave, is it valid for me to leave over something that I've known for a year now, sometimes I think I should've left sooner but also i know we are in a good place currently, and if she's the one who's lying surely then she's won. We also decided to have our second baby once we'd cooled down after this.

I also said to him I don't want us going to her house to pick up the kid and he needs to find a way to see her without us having to see her mum at the same time this resulted in him not seeing her for a good few months as he proposed pick up and drop offs from granny's or through a middle person( she said no) he called her controlling and blocked her a few months later he came to me saying he misses his daughter and wants to see her, I felt horrible but also didn't really have a choice if he wanted to see her, it gives me literal shivers driving to her house and all the overthinking started when he went back to seeing his daughter, she refuses to let anyone other than dad pick her up even tho im struggling to fit 3 car seats in my car, she refuses to drop her off either.

What do you think? Should I leave for something I found out over a year ago and that happened 3 years ago, should I have left sooner? Or is it just me being paranoid and overthinking with the stress of a 2 month old baby