Came back about a month ago from a live aboard trip that was 10 days. The first day we didn't dive, we didn't get to the boat until after 5 PM, we were able to dive 5 times a day except for 3 days and only 4 dives were offered and the last day of diving only 2. I got in 36 dives in 8.5 days of diving. 39 were offered and I missed 3 night dives. Hell yes I was tired in the evenings.
I paid for Nitrox, I think it helps me. Drink plenty of water, eat well and back off the beer in the evening, go get a good nights rest instead. On the boat they fed us a lot and we ate a lot, lots of calories, snacks between dives good meals all.
I'm 60+ years old and would love the chance to do it again. After 10 days I was ready for some rest but never had problems with feeling exhausted. Watch your dive times and NDL's. You don't have to dive to 100' every dive. Shallow it up and do 5 or even 10 minute safety stops. Don't be in a big hurry to cover a lot of ground. Relax, slow down and enjoy. If you are too tired to make that 5th dive, don't do it. If an old guy like me can do it.........