I agree with the advice already gives, and as you have discovered, pushing yourself can make a huge difference. Take a look at the re-row feature on your PM4 to race against your previous sessions. That pace boat can be a great motivator.
I'm really surprised that this hasn't been mentioned yet, but have you experimented with different drag factors? If you are lacking in strength as you suspect, you may see an improvement by lowing the df slightly. Try a few sessions at 115 and see what it feels like.
You may also want to experiment with your stroke rate. I am much heavier and taller than you and will average 26-28spm for a 2k with peaks in the low 30's. 22-26 seems low for a lightweight with short legs, but stroke rate is a very individual thing, so you will have to find what works best for you.
