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Not being able to assert your will can make you susceptible to ...
Does "assert your will" mean the same as "express your will"?
Not really. You might be able to express your will (say what it is that you want) but still not assert it. If you assert your will, then you actually get what you want, not just say what you want.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.