I wonder if she particularly wanted her husband or the lifestyle & friends?
I think this is what it is - she has a terrible inferiority complex - wonder what her background is ? - all she's had is her looks, and then "Jeff" (her ex) shags her chiropodist and dumps her! All she has ever had is "things"that she achieved by being beautiful enough to catch a rich husband (or perhaps he wasn't when she married him - maybe she's helped to make him his millions and now she's been tossed away the he doesn't want or need her any more).
Derek never meets Jeff does he? She can spin any sort of line about him - own business, worships the ground she walks on etc - in an attempt to bolster her status in her ex's eyes. Or maybe she really does love him (Jeff) and is desperate for him back. Or maybe she wants him back so she can dump HIM.
Derek is safe - she will never have to worry that he will leave her no matter how she abuses him (and this in itself often starts a cycle of ever-increasing abuse - I don't envy Derek's life with her).
Kelly, I think, has been led to believe that she is thick and ugly - her ghastly mother (who also has huge emotional problems I suspect, which she hides behind her armour of "funny girl") has led her to feel like this (we never hear about Kelly's dad - I wonder what he did to the family). As she gains confidence and matures, Kelly becomes much more self-aware, and I think she will one day find that she isn't stupid at all - intellectually she is far above Jason, and in ten years time may find she can't live with him - or she could become another Cathy - sublimating her own needs and desires because she feels responsible for her family.
Jason, alas, is as thick as pigshit but I think he will become an emotionally more mature person and learn to consider other people's feelings - I really think he'll do his best to be a good dad, and that because his own dad isn't there to interfere might make a good job of it.