Damn son. I never even considered the miscarriage/losing a baby angle. Yeah, thats the sort of thing that'll fuck up a woman for years. I like this theory.
Personally, I think there are a few things that don't quite add up with this theory being the sole reason.
For starters, there's Bella saying to herself in Ep 3, "Please, James... Forgive me". Now, could she blame herself for a miscarriage and this is what she's asking forgiveness for? Sure, however, this line is preceded by the lines, "They aren't intimate with friends either... // But that...that was different." When taken together it sounds like Bella is asking forgiveness for doing something "intimate" with a friend that drove James away. Some people might think that "friends" refers to either Jade and Stephen or the MC, but we know that Bella didn't accept Burke's offer for the threesome with him and Jade, so that rules them out as possible "friends", and at this point I would say that she doesn't yet consider the MC as a "friend" either so that probably rules him out too. Plus, her saying, "
that was different" makes it sound like it was some time ago and is not something she did recently.
Secondly, when you stay with Bella in Ep 4 and take things further in the pool, she then says of James, "I have forgiven him..." So now it would seem James also did something that Bella forgives
him for and again those thoughts are brought about because of how it links to her being intimate with someone else. Prior to that, she also thinks to herself, "Would he blame me if I did this? // After all this time?" This likely relates to her need for intimacy having been absent it for so long, also the implication of her wondering if
he'd blame
her makes it feel like he was guilty of doing something similar and thus shouldn't blame Bella for doing the same, lest he be a hypocrite.
Then there's the whole Jill/Bella thing in Ep 5 that possibly indicates something happened between them, and Bella in Ep 2 after kissing the MC, says to herself, "How long has it been since a man did this to me? // Three years?", which would seem to very clearly indicate that she hasn't felt such desire towards anyone, at least "a man", for 3 years. Now, if she had had a miscarriage 3 years ago, she may have lost her desire for sex which would explain this line, but equally she could have lost her desire if James cheated on her 3 years ago, and although she still wants him back, she didn't feel much desire towards him after that because of the betrayal.
Maybe the cheating and miscarriage theories are separate things, or maybe they could be one and the same; Bella had a miscarrriage, James had an affair as a way to escape his feelings, they tried to work things out, Bella also cheated on James briefly, and then they separated. But, I'm not so convinced that it would just be miscarriage on its own especially given the several hints towards infidelity.