My husband secretly texted his Ex while I was having our baby

So I recently found messages that my husband intentionally hid from me.

For context: I say “ex” but they never dated. Him and this girl had sex together before him and I met. Shortly after we started dating him and I talked about everyone we ever slept with so I’ve always known about her. We have been together for 6 years now, married for 3 years and recently just had our first child.

A few days ago I was sending myself photos I took from his phone when I saw this girls name in his messages. I clicked on them to read through and it turns out he has been texting her in secret for months or maybe longer? I have briefly told him in the past that I was uncomfortable with them staying in touch because I don’t see a reason for them to. I was under the impression that this was just a casual fling and that she is not a significant person in his life so I see no reason for them to continue to communicate. I never gave it another thought. However, these messages felt very condescending towards me when they talked about how they both think it’s silly that I won’t let them stay in touch. He even asked to meet her at a coffee shop without me knowing and asked multiple times if she was available for a phone call. He also told her to start messaging him on Instagram because it’s more private and I’m less likely to find their messages there.

When I found these I immediately confronted him and he played dumb at first and then he said that they were close friends for years before he met me and that he didn’t like being told that he couldn’t talk to her. He swore nothing happened between them since we have been together and that they never actually met up or talked on the phone. I just don’t know if I believe him? And now I know that he has no problem lying to me and hiding things so it has me questioning if I can trust him at all?

Another massive issue is that during the time span of these messages I was 9 months pregnant and then I had a c section and the recovery was brutal. I struggled to be able to walk for days and then I got a horrible allergic reaction to the bandage on my scar. I then got back to back mastitis 4 times and had a fever of 104°F. THEN I got COVID. It was absolutely horrible and I don’t feel like he was there for me at all. He didn’t help me with nights and he always complained about being tired the next day. When I had a fever he took a nap while I took care of our newborn son and told me to just wake him up if I felt like I needed to go to the doctor. I still have a lot of resentment because of the lack of help I got during my postpartum and now I find out he somehow found the time in all of that to be talking to this girl?

I have no clue if this is something I should work through or if it’s a warning sign that this marriage is doomed??