I experience that as well, the voice gets kind of hard to control, and after some more challenging songs, its like I don't have the stamina to hold the higher notes.

I've always thought of it as a sign that the excercises are doing their job of strengthening the muscles.

Kind of like when going to the gym. You wouldn't enter a competition right after an intense workout, since you're already exhausted from training. But after a good rest, your body has recovered and you're in better shape than you were before, thus building stamina.

I might be way off though, but in my head, it makes sense, since we're still talking about muscular excercise, only it is the muscles used when singing that's exercised.

Any thoughts?

I like the weightlifting analogy however, I don't usually find you need as much time as what you would with lifting weights (day of rest etc.) Leave a gap after your singing exercise before singing the songs. I'd say leave 20 or 30 minutes and then come to the songs. You will have the warmup from doing the exercises and singing will feel in gear as opposed to being cold and unprepared.

Yeah i find problems with stamina. I must admit it feels like i am not supporting when actually i am pushing down so hard. I can feel the effect on the muscles behind the chin for sure. I guess they are still not strong enough.

I think that making sure the diaphragm is not pushed under the ribs is pretty important though. I think the pushing down engages the diaphragm well if the diaphragm is in a good place to start with, otherwise you feel like pushing down isn't giving you much support at all and you're simply tensing your anus for nothing.

Make sure that your belly comes out well when breathing and that you can feel the muscle beneath your solarplexis well then apply the downward support and you will feel the diaphragm engage and take tension out of the neck and throat.