Spend some time daily on an erg working on technique - in particular making sure you are getting the right sequence (basically legs, body, arms) and working on rocking from the hips with a straight back.
Video yourself erging so you can see when you're getting it right and also get a coach / cox / experienced rower to give you a bit of coaching on the erg.
Get the technical changes bedded in on the erg and then work at translating them to your single.
Also look at doing some core work / yoga / Pilates and maybe some weights to support and strengthen your core muscles to allow you to hold the technically appropriate positions.
have someone look at your posture. do you curve your spine a lot before you pull? you need to keep your back in a nice stable s shape. harder work in the beginning, but you back will thank you. also do exercises to counter the back muscles, for example hang on a horizontal bar and lift your legs a little, try not to twist your torso doing this.
I'm in my 50s and started rowing (sculling) last year. I row once a week and love it, but my lower back aches for days afterwards and I feel as though I need to walk in a slightly bent over position. (Not a good look).