Colloquial alternative: Ek bemin en daarom bestaan ek. *derhalwe* sounds somewhat outdated, at best it is "officialese". The common word for därför/therefore is *daarom*. However this could be misconstrued as indicating causality. Obviously, in this sentence därför is equivalent to Latin *ergo* or English *hence* (indicating a logical conclusion), which in my opinion is more accurately rendered by *derhalwe*