If you are looking to produce professional quality voice recordings (for example, audiobooks), you need to fix the problem at source - practice maintaining a consistent delivery in the way that you read.
If you don’t need professional voice recording quality, but just need something that doesn’t sound too bad on YouTube, then you could try using the “LevelSpeech2.ny” plug-in (see this post: Improved "leveling" for speech - #27 by Trebor).