I'm sorry to be Captain Hindsight, but a 26-year old man should not get in a serious relationship with a 33-year old woman. You were just starting your prime, and she was well past hers. The good news is you are still in your prime.
Get DNA and STD tests.
Your wife sowed her wild oats with hot bad boy types. Unfortunately those types tend not to provide the stability and security needed for a family. Female fertility starts dropping around age 30. She needed a nester nice guy like you. Those qualities were what she found attractive in you.
Think of her as your teenage daughter who was dating a bad boy you didn’t approve of. She had you for security and stability and the OM for fun. She’s upset because she doesn’t want to lose what you provide. She may “love” you but it’s like loving a family member. The only leverage a dad has over a teenage girl in heat is to let them know that they can be kicked out of the house. (No more security from dad). She will tell daddy whatever it takes to keep her room then sneak out of her window at night.
This relationship had so many negatives right from the beginning. With her biological clock ticking LOUDLY, she grabbed an opportunity to have a kid with a guy who could be "marriage material " even though he was not her type. She was able to suppress her desires for a short time but with douche bag next door, she couldn't restrain herself.
As a mid 30s, never married woman, she probably has been riding the carousel for quite some time before meeting you and has learned quite a few tricks that can ensnare a man. I bet you had very little experience with women and were probably quickly pvssy whipped when she busted her moves on you.
Problem is that if a wife isn't strongly attracted to you, it doesn't take long before her mind starts wondering. You said 2 of her past boyfriends were strong fitness types and then when she finds herself with a neighbor that is her type, she can barely contain herself. Even with your child in her belly, she's fantasizing about POS and QUICKLY starts her action plan to hookup with him.
To top it off, you suppressed your career so she could further hers. This caused her to lose respect for you. In her head, now that she achieved some success in her career, you're beneath her or at best her equal. Women want to look up to their man.
You may find that you're a better man and father without an emotionally draining woman.