For me that's the situation with lesson 3 ex 5 of the ditones book which is call "add".
The exercise is a 115 bpm one, going vertical with 4 digits ditones (see the ditones how to), each time adding a string (5th and 4th; 5th, 4th, 3rd; etc). I'm stuck at around 100-108, dipending on the days, even if I am able to play the next exercise "pelota" - 3.6 - that features some speed bursts at 120).
I play it slow everyday, practice with care but, no matter what, I won't get it at 115.
There's some psychological trick involved here.
So, what can we do about that?
There's no bpm you can't beat.
The hard part is reaching a certain bpm with your hands doing a certain exercise. Here's how the psychology goes. Take an easy example: 200 bpm. A pretty fast thing, though.
1) You can "beat" 200 bpm playing quarter notes.
2) You can play eight notes with some practice.
With some practice you can
3) Get stright 16th notes on time is more tricky, but you can still do it...
4) Nailing down a 1234 exercise rather than a simple empty low E string can still seem impossible...
But, wait! There is always a way to play a certain bpm. Steps 1-3 proved you that. It is not that 200 bpm is too fast for you, there's no tempo you have to fear. 200 bpm is now too complicated for you to get 16 notes 1234 in an accurate and coordinated way.
So you can start working on 1234 at 100 bpm or 120. You may then reach 140 bpm with is something that allows you to play the theme from this video...
Turning to practical things, this mean that in order to avoid the "here I am again at the bpm I can't play" panic or psychological breakdown, you have to start feeling confortable at that bpm or even higher. That's a list of the things to do:
1) Just play a simple 8-th notes rhythm, then try a few licks on that beat, play quarter notes, eight notes and 16-notes and get confortable.
2) Then play some easier ditones, such as that on a single string and one on two strings.
3) Now go back to your exercise and you'll be surprised how much more confortable and more relaxed you feel. The beat won't seem to fast and no good for you.
So you only have to coordinate you picking and fretting hand at that beat. Good luck!
Remember to coordinate with that blog and subscribe. And please comment telling how your ditones journey is going!