Am I being paranoid, or does my husband's relationship with his coworker sound suspicious?

My husband has recently decided that he wants to end our marriage, and I am struggling with the feeling that there may be someone else.

For context, last year I was actually the one considering ending our marriage. He was the one who wanted us to stay together and give the relationship another chance, so we did.

And from my perspective, things actually seemed to be improving. I genuinely felt like our relationship was getting better and that we were moving in the right direction. Obviously I can only speak for how it felt to me, and he may have experienced things differently, but that's a big part of why his decision to leave has felt so sudden and confusing.

A few months after we decided to stay together, he became increasingly close to a married female coworker/friend.

They started commuting to work together for practical reasons. They regularly have lunch together at work, talk a lot, and started going to the gym together.

The gym part stood out to me because my husband had always struggled to establish and maintain a consistent gym routine. Suddenly he was going regularly and sometimes spending around two hours there with her.

During roughly the same period, he started paying considerably more attention to his appearance, buying new clothes/underwear and generally changing some of his habits.

Then, while I was thinking that our marriage was actually improving, he told me he wanted to end it.

I've asked him directly whether there is someone else, and he says no. He also says this woman doesn't even know yet that he's leaving me.

The biggest reason I'm struggling to take his word for it is his history.

He has cheated on a previous partner. He also previously managed to maintain relationships with two women at the same time for around a year without either woman realizing there was someone else.

So when he tells me, "There is nobody else," I'm not dealing with someone who I know would never be capable of hiding an affair. I know from his own history that he has been capable of maintaining that kind of deception for a long time without being caught.

At the same time, I don't want his past to make me automatically convict him of doing something he may not actually be doing now.

I know that going to the gym, having lunch and commuting with a coworker don't prove an affair. It's the whole pattern that is bothering me: the increasing amount of time with her, the changes in his habits and appearance, his history of cheating, and then his decision to end our marriage at a time when I genuinely thought our relationship was getting better.

I'm also wondering whether I'm looking for the wrong kind of affair.

Maybe nothing physical has happened. Part of me wonders whether he has developed feelings for her and is ending our marriage hoping that, once he's single, she might eventually leave her husband too. If that's what's happening, I don't know how I would ever prove it.

I've even considered hiring a private investigator because I feel like seeing something unmistakable, such as them kissing or holding hands, would finally give me an answer. But I'm also aware that I may be going too far down the rabbit hole trying to get certainty.

So I would really appreciate an outside perspective.

Does this pattern sound genuinely suspicious, or am I connecting unrelated things because I'm hurt and my marriage is ending?

Given his history, would you believe him when he says there is nobody else?

Does this sound like it could be an emotional affair rather than a physical one?

And is considering a PI completely excessive, or would you also want independent confirmation in this situation?

Please be kind but honest. I'm not looking for people simply to agree with me. I genuinely want perspectives from people who aren't emotionally involved in the situation