If I’m playing the octave D and I go to low E, my note seems to always want to “break.” Is there something that can help with this problem, or is it just a question of breath control?
Although no expert i think breath control is very important here.
I assume when you say “If I’m playing the octave D” that you mean the second octave?
Do you vent the top hole when you play this D? Sometimes it can be easier not to vent the top hole with certain finger changes.
Some whistles respond differently to others, so it’s worth trying both.
I’ve just tried this change on a few of my whistles that have different characteristics, some had a less pronounced start of the E note. I found it helps on some whistles to slightly tongue the E to give it a crisper start, It really depends on what tune you are playing.
I’m sure there are other more experienced players who could say more here
Thanks.
The tune is Drowsy Maggie, so yes, it’s the 2nd octave D (The first measure).
I’ve tried both venting and not venting. On my Susato wistle the problem is worse, and venting doesn’t seem to help. On my Meg wistle it’s not quite as bad, and venting seems to help a bit. I’m just starting back at whistle again, so I don’t have things totally under control yet, if you know what I mean 
You can emphasize the E with a cut