I have to say this doesn't sound good. I know that its very hurtful to see her doing these things. I would say you have a two phase issue.
Now, this first phase I'm going to recommend does not mean you are to blame for any of this. There very well may be something going on. It will just help to limit accusing her and pointing fingers and concentrate almost entirely on how you feel. There are some real red flags here, but just hear me out.
I think the first step is being honest with her again that you're troubled that she's spending so much time with B. If she tries to tell you there's nothing to worry about, tell her this is not about controlling her behavior but about feeling disconnected from you. Tell her it would help to try to re-establish a connection. If you wanted to push the issue in a more direct but respectful way, you could say something like "I know you've been spending a lot of time with him, if there is some need that he meets that I don't then give me the chance to listen, learn and grow and meet those needs". You can tell her that even if it's totally platonic, getting her needs met through a relationship with another man is a recipe for trouble. Again, don't give in if she tries to tell you it's nothing. Don't drop the topic and stay consistent and calm. Don't talk about what she does , like "You go out with him", "you flirt with him", etc. Just say you are trying to be the best husband you can be and it's important that she understand that you feel disconnected. You can get your point across without even mentioning B more than once or twice. If you feel yourself starting to accuse her, or badmouthing B, step away and collect yourself. You have every right to be upset, and quite frankly there may be something going on, but the whole conversation will start to go downhill if you are negative towards her or B. If you are persistent and keep focusing on how this is making you feel, you may find out the truth.
If she seems to respond and the red flags start to go away, become an expert in what fills her tank. If you haven't already read the book The Five Love Languages. Figure out the top 2 or 3 things that fill her tank and do those every day. Secondly, think hard about things that she tells you she doesn't like you doing - even ask her what bothers her about how you act - and stop doing those things immediately. Love and connection is like a bank account - loving acts fill the account, disrespectful and hurtful acts drain the account. When the account gets low it's bad news for a relationship.
On the listening thing, you're really going to have to work at this. She can tell you're not really tuned in when she's sharing her work stuff with her. I can tell you I have a female friend in the same boat - she's an office worker and her husband is in the trades. She tells me she can tell that he's not really listening and only pretends to just to keep the peace or get sex. I'm not saying that's why you do it but I can tell you that my friend is hurt by the fact that her husband isn't showing general interest. You're not a dumb guy, you just don't understand the lingo of her work. So what's the best way to show interest in that case? Ask questions. Ask what the words mean, if something is making her upset at work, ask what specifically about it gets her riled up, and then give her genuine support when she tells you. Use validating language "I can totally see why you would feel mad when Fred didn't get his reports done on time", etc.
I'm sorry you're struggling with this. Everyone has areas they can improve, but you don't deserve to be treated this way. I want to repeat, I'm not blaming you for any of it. I just think if you calmly focus on how you're feeling and keep the anger and accusations to a minimum it will get you a lot farther, even if something is seriously wrong. Good luck.