I'm gonna take something from my trumpet playing here. I've been playing trumpet for seven years now. And as a person who can play any wind instrument, and is fairly proficient on bass, I can say that trumpet is HARD. DANGED hard. The human mouth is simply not designed to do that. And here I am, trying to force it to do it, anyway.
This is what my teacher says over again: you build up your trumpet house one brick at a time.
Now the speed we're talking about in bass is an athletic thing. It's about your muscles, not your music. In trumpet, the athletic thing we all strive for is the high notes. But my teacher, who is an excellent teacher, has never had me do a single high note excercize. I play my long tones, my scales, my lip slurs, my tonguing excersizes, etc. I have never just sat there and played high notes.
And, well, I know it sounds arrogant, but I'm a senior in high school and I'm one of the best my school has ever seen.
It's the same for bass. When I started, I couldn't come close to playing with one finger on a fret. 5 years later, I can do it easily even at fret one. Have I ever done "stretching" excersizes? No, of course not. It just happens.
So keep practicing.