I just want to recommend a presentation from GOTO 2018, called “Modular Monoliths“. In the presentation, Simon Brown talks about software architecture and modularizing monolithic systems.
Rewriting an existing monolith into a microservices architecture, can be both risky and expensive, and the end result might be just as bad, or even worse than what you started with. A better strategy might be to start by modularizing your existing monolith, instead of trying to replace it completely and go full microservices at once.
You can also take a look at the slides from the presentation here.