。
I've decided to keep doing this job.
Literal
I [topic-は] this job [object-を] continue [thing-こと-に] decided.
~ことにする is the standard 'decide to [do]' construction. こと nominalizes the verb phrase, and に+する means 'make it [thing/state]' — together 'make it the case that I [verb].' Distinct from ~ことになる ('it has been decided' / 'it turns out that'), which is passive/circumstantial.