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I posted earlier and wanted to add something the LW should definitely be asking herself, since she's considering marriage...
Would you want the man, you claim to love, be the father of your future children? You already know that he does not care about his current child. That will not change 'just' because you'll be the mother. Are you willing to gamble that he 'might' improve?
Or, are you considering to remain childless yourself? Which would make sense from your impatience expressed in your letter.
Raising any child - regardless of (dis)ability - is time consuming and takes a lot of patience. A lot of the things you describe and take for granted as 'common sense', need modeling and consistency. Anna has had none, and you haven't been willing to be the role model.