Complex sentence: Arabella feels that she cannot make a good match when all the town falsely believe her to be wealthy and knowing that Mr Beaumaris can have no designs on her supposed fortune and is only amusing himself with her, she is most comfortable in his company.
Split into simpler sentences: Arabella feels that she cannot make a good match when all the town mistakenly believe her to be wealthy. Knowing that Mr Beaumaris can have no designs on her supposed fortune and is only amusing himself with her, she is most comfortable in his company.