I honestly can't blame the FL for neglecting her son. Sure, her son is an innocent - but she was impregnated against her will at age 17, she then (unsurprisingly) suffered from post-partum depression, and then on top of all that her husband (who her son resembles greatly) kills her cousin? You'd have to be a literal saint to work past all that and act like a nice mother. At, again, age 17. A literal teenager.
It's ridiculous that people will easily forgive the horrid dads in the series where the MC reincarnates as their daughter, usually after the horrible dad in question literally kills them. But when a woman can't be a good mother after she was forced into a teen pregnancy, suddenly she's unsympathetic and unforgiveable.