After the unexpected death of my first love a few years ago (we had broken up decades earlier), during some grief-filled reminisces it became clear to me how as inexperienced teenagers we are expected to make adult decisions that will effect the rest of our lives. Of course, at the time we don't worry about it since teenagers think they know everything. We don't heed or need any advice from adults since we think we know what's right for us and want to live our lives our own way.
Poor headstrong teenage Catherine got all starry-eyed over the idea of being the lady of Thrushcross Grange, capriciously ordering around a crowd of servants, wearing expensive clothes and jewelry, having great status in the village. Edgar was pretty cute too and bent over backwards to make her happy. How could she know that life is full of yearning for whatever we don't or can't have and you have to learn to prioritize and choose only the things your heart and soul really need because you just can't have everything. Catherine truly believed that she could have everything. Besides, Heathcliff had been gone for a couple of years without a trace. For all she knew he was gone forever making the decision to marry Edgar that much easier.
After Heathcliff's return she still believed that if they behaved as she wished, she could have them both. When she finally realized she couldn't have both Edgar, her husband, and Heathcliff, her soulmate she did what a lot of teenage girls do and locked herself in her room and refused to eat. Teenage girls are really good at self-destruction. I like the fact that she was five months pregnant at the time making it impossible to blame it on PMS. She was just a teenager who couldn't get her way.
When she finally let Nelly in after three days isolation, she was a physical and mental wreck. She realized what a mistake she had made marrying cute, but boring Edgar and longed to rewind the clock and be back at Wuthering Heights traversing the moors with her heart's desire Heathcliff. Now she was trapped in a passionless marriage with a baby on the way. Marriage was for life back then even if you lived apart like Isabella and Heathcliff, so Catherine's life was over as far as she was concerned and she looked forward to her physical death to escape the pain of never being able to live with Heathcliff at Wuthering Heights again. Without Heathcliff she was no better than a dead woman walking.