“Hi Philip, I am performing a concerto for the first time ever in a few months, and I’m having trouble with my stamina. I can get through it, but by the end I find that my tone isn’t all that good because I’m tired, and my breathing goes out the window. I was wondering if maybe there is a better way of practising? Should I be working on my breathing away from my instrument? Thanks Philip.”
I understand that feeling very well! It can feel like you just want to stop playing and walk off stage but you're trapped. There are so many factors that can add up to that feeling of exhaustion - taking too much air onboard, playing on reeds that sound good but are too hard and so require more air and strength, playing on a setup that needs too much effort to produce volume in the first place, playing too intently early on in a performance rather than pacing yourself etc.
About your tone, have you tried recording yourself? You might be surprised to find that it probably doesn't sound so different than it did at the start. Your tiredness and feeling of lack of control might be contributing to your perception of how you sound. I think your focus needs to be on the music and what it's saying at the end, it's no time to be self-judgemental!
A good way to improve your stamina is to take up swimming. The activity encourages good air intake and release under exertion, whilst being good for the back and arm strength you need to maintain good posture.
Practising away from the instrument is also a great idea. Practise taking good, less noisy breaths and then gradually practise making these quicker.
When I'm struggling with breathing in a particular passage or piece I try to fix it by stopping at every breath point, exhaling all my bad air, taking a relaxed slow breath then continuing. This stop can be 2-3 seconds and I try to breathe at the same points each time. Closer to the performance I start shrinking this 2-3 seconds gradually down to whatever the rhythm requires. This method should help you considerably!