Sounds to me like you have one of two problems. You started in to this program fatigued from what you have been doing, I.E you need to take a extra day or two and rest. Or it's your conditioning. From the symptom you described it sounds like fatigue. Take an extra day off, go do something fun and try it again.
I tried the circuit training like Chad described and it kicked my ass, but not only did I do the circuit, I did two of them. Unfortunately, I was running low on time for the second circuit so I chose a 5 x 5 methodology to get through it a little faster.
My first circuit was heavy as fuck pulldowns w/ neutral narrow grip, then hanging snatches into shoulder presses, then heavy, frog stance leg presses. The second circuit is strait-leg dumbbell-deads, bent-over dumbbell rows, and bench.
It kicked my ass but I did it, and I am going to do something similar to that today. I am 35 and I can do it, so can you.