If you record yourself say two sentences and play them sequentially, this would sound unnatural as the end pitch of the first sentence would likely differ from the beginning of the second sentence. Pitch adjustment is banal in the music industry. Is there a pitch adjustment tool that solves the problem of merging sequential recordings?
Voiceover, like any recording activity, is a skill. One built up with much practise.
If you're recording yourself, then it's two inter-related skills.
You can practise this by reading a book or your script out loud. Consider your pacing, breathing, diction & projection. Even if you are unskilled, your voice will tend towards a more relaxed, consistent & easily-repeatable sound after perhaps half an hour of reading.
The beginner's problem is usually that they're too eager to want to get on with the recording & don't allow this warm-up time. Doing that, each take or drop-in will sound different to the last, making the edit a nightmare.
Whether from a book or script, run through it for half an hour, as you're setting up the mic & getting levels. Don't even bother recording the first few takes.
One you think you're warmed up, do two or three read-throughs as takes [make allowances for the duration of each take, of course, a 30-second advert you could do ten times in a row. A full novel… perhaps not so much;)
Once you've got three takes, then compare by flipping the mutes between takes; first to second and first to third. If first and third don't sound similar, you have to discard the first take & go for another. Only when first & last sound the same are you in a good position to edit between them.
The difference between the amateur & professional in this is simply the length of time it takes to achieve consistency, from first walking in the studio. Both can get there eventually, a pro will just be faster, because they're more practised & have learned how to make their voice give a specific performance style, on demand.
As your own recording engineer, part of your warm-up process is additionally to get your mouth-mic placement exactly the same each time. Different distances & angle of incidence will produce different sounds, even if the voice is the same each time.