Complex sentence: Lawrence was also convinced that the American government owed him a large sum of money but that President Andrew Jackson was keeping him from receiving it, which prevented Lawrence from taking his rightful place as King of England.
Split into simpler sentences: Lawrence was also convinced that the American government owed him a large sum of money but that President Andrew Jackson was preventing him from receiving it. According to some accounts, Lawrence blamed Jackson for preventing Lawrence from taking his rightful place as King of England.