Am I just thinking bad?

Sorry for the long post! I really need advice and different perspectives please! Husband (37m) and I (39f) have been together 13 years married for 11. We aren’t on the best terms. We were doing couples therapy but after a few weeks he kept saying it was just a waste of time! He has anxiety and has said that it’s now worse- but still doesn’t want to look for help.
He says the only thing that “helps” is drinking (every weekend) even though he knows it’s unhealthy.
About 3 weeks ago, I wake up at around 2:30 am and see that he’s not in bed. I look for him and find him outside in our side yard where we have our bbq set up. He doesn’t see me and I notice that he’s on the phone having a video call. I hear him say “I feel close to you like this too” and then made a silly/flirty face (one that I know all too well).
Once I saw that I asked who he was talking to. He said his mom… how I remained calm I don’t know but I said ok. Went back to our room and called his mom which she confirmed my suspicion that she wasn’t.
I walk back outside and he sees me coming and immediately gets up and is almost running to the backyard. I ask where he’s going and claims that he was going to urinate! I try and grab his phone while asking who he’s talking to he again says his mom. I tell him that I spoke to his mom and she said he wasn’t talking to her. He says call her which I did and he tries to say “mom wasn’t I talking to you?” His mom says that it’s not true and that she had already spoken to me.
I find that he was on WhatsApp. I tell him to open it and he wants to take the phone. After, arguing for about 40 mins I give him his phone and proceed to delete everything. He then lies 2 more times about who he was talking to. I now know they are lies because a few days later I finally open his WhatsApp and see the call log. All with the same woman. I ask who it is and says it’s his cousin but her brother uses her phone and that’s who he talks to.
Days after that he finally “admits” that he was talking to her.
I ask why did you delete everything that only makes me think that it was something inappropriate.
I saw he sent selfies I’m not sure if it was to her but I doubt he’d send selfies to a male cousin.
He says there wasn’t anything wrong with what he did and talked to her about. Said I should be thanking her because he felt good talking with her and she’d give him advice about our problems. I’m not sure how WhatsApp works but the oldest phone call was from March.
They were close growing up but after he turned 14-15 they stopped visiting and in my whole relationship with him I never once heard anything about her or her brothers.
I feel betrayed and I don’t trust him.
Am I overthinking? Is this normal for cousins? Am I just thinking bad because of the situation we are in?
I’m so confused and honestly disgusted!