The rod shouldn't make a difference in wearing you out. Work with a casting instructor and figure out what the issue is. I can help you depending on where you are. Or shoot a video and send it to me or post if you prefer.
And I wouldn't invest in a "used" rod unless you know that you love it or are willing to take a chance.. Most used rods are being gotten rid of because they didn't work for whomever is selling it. Doesn't mean it won't work for you, but you need to cast different rods and figure out what rod and line combo works best for you.
I'm going to get a couple of Winston's next, a 7wt to start and maybe a 4wt down the road. I don't really need any rods as I have 30 or so rods anyway. The other two I plan on is a 13' bamboo spey rod from a guy in Idaho, and is more than I should spend... i won't even say how much it will be! And I want to take Bill Oyster's class in Georgia and build my first bamboo rod.. probably a 7' 4wt.. it's a week long course, where you build and wrap the rod and then fish for a day with it when you're done! With Bill and the class I assume.
Good luck with whatever you end up doing!