I have found myself in the same boat last couple of months. The power I had (and there wasnt much to start with!) has somewhat evaporated.
The pace I could generate at r20 is more like r22 now. I am carrying the usual niggles to work around but I am not using this as an excuse.
I think I have been doing too much slower stuff this last couple of months and need to mix it up. I have mostly stuck to hour sessions but although these aren't generally all at one pace, the average at the end is slower than I was doing before.
This summer I also have done nothing else but row on the erg with a few walks thrown in.
I have also been playing around with HR workouts since getting a HRM.
I don't have any athletic or sporty background. I was probably at my fittest at 25 but that's a lot of years ago now! I was the kid getting kicked up the backside during cross country at school (when the games teachers were allowed to do this!), last to be picked and so on. Soon as I got into 6th form and could go and play squash, badminton, go sailing etc I was off!. So this erg thing and the performance of where I sort of am now I have largely accepted will probably be as close to where I ever get to (although of course I would like to keep improving I know my curve has flattened out to almost being horizontal

)
So my plan is
Ditch the HRM. I don't need it. I know what I feel like and sometimes this it at odds at what the HR is telling me.
Go back to the workouts I was doing before with harder interval sessions in between the SS.
Try to mix up the rowing with other sessions and try to introduce some light weights again niggles permitting.
Overall train smarter without putting more hours in.
Go back to looking at where I am on the percentiles again for age group and be happy to gain a few places rather than look at absolute times.
Sleep more - not an easy one to fix.
Also eat more!
Re this last point - I don't think anyone else has mentioned it, but 154lbs seems very light for a 6' frame? Have you lost too much weight?
Good luck with deciding course of action to break out of where you are now.