Stumbling over yourself and crashing and burning whilst coming out of a bass fill is not a good look (or sound). For beautifully smooth transitions from fills to bass lines you need to learn a few basics including:
Learn how to dominate your bass fills by watching this video and then practising with the backing track, shapes, and chord progression below.
F Minor Pentatonic Scale
The lesson demonstrations are all based around these shapes. You need to know them inside out and back to front.
Note that if you want to play the scale up the octave, starting on fret 13 of the E string, just play the same shapes all over again. For a deeper dive into the minor pentatonic scale, study these lessons:
Here's the chord progression for the backing track. It's in the key of F Minor and all the chords belong to that key. That is why F Minor Pentatonic works in this situation (and so does F Natural Minor if you want to give that a go too).
The blues scale is really just the exact same collection of notes with the addition of one note (the flat 5). That's a great one to add an earthy, roots sound to your fills. The combination of pentatonic with blues scale always sounds soulful and sophisticated!
Follow the link below to learn more about the blues scales (there are two main ones - minor and major). For now though, just add a note between any Bbs and Cs from the diagram above and voilá: you have a blues scale.
There's no doubt that you have to get on your bike if you want to execute an accurate fill. Sometimes you just have to move your hand quickly to get back to beat one of the next bar (by the way; that's more or less the most important thing to master when it comes to bass fills).
I cover hand shifts and many other bass techniques in my book Creative Bass Technique Exercises. Find out more here:
Remember that what you deliberately focus on in your practice sessions will transform something from difficult and unfamiliar to effortless. Make sure you're always selecting areas of your bass playing to improve. Live with that thing for a few weeks and then move on to a different area. Always do this and you'll get better without you even realising it.