Learning to fly is totally, utterly, and completely knackering. You're just normal!
As Gemma said, instructors don't know how much you can take in, as people vary dramatically. Because of the cost, and because a lot of people who learn to fly like challenges, we instructors tend to push students quite hard. Often though, we don't even know we're doing it. I often don't realise students are tired until they start to move the controls the wrong way when previously they were doing it right, or don't hear what I'm saying to them even when I yell. At that point, I tell them they need a rest. What usually happens is they swear they're OK and not a bit tired!
As of the end of next week I'll be starting 2 hours a day, 5 days a week, hopefully that won't make things worse, should be getting used to it with those kind of hours I would imagine.
You may find that doesn't work for you. Some people can cope with intensive courses, some can't. Personally, I hate them. I tried to do my PPL that way and it was completely counter-productive - I got so tired I couldn't learn. I abandoned my CPL in the middle because by then I knew enough to know it wasn't working for me; I had a break and then carried on. So try by all means, but you may find that isn't the optimum way for you to learn. If you have a good instructor he should notice and tell you, but don't rely on it - you'll need to decide for yourself.