My advice would be to get a heart rate monitor. You can get affordable ones, $40.00, you don't need anything super fancy!

Going by feel is great, once you know what you are feeling, but if you are just going by feel without having actual numbers to compare to- you have no idea what you are doing!

There is a good chance that you are working a lot harder then you think you are, or you do just need to lower your resistance. You can only push a big load for so long before your muscles fatigue.

I advice getting a HRM so you can actually see what you are doing and when, and monitoring your resistance and trying to keep a higher cadence.