Painters tape and a specialized roller. Ask your local paint store. Do your best but don't obsess over it. Everybody knows it's an old house with quirks. That's what you love about it right?

I agree with nomimama about using the small brush. My house is 1920s with NO straight lines anywhere, but I have had pretty good luck painting edges with the small brush, and keeping a small damp rag to hand to wipe off any careless oops immediately.

Small angled brush... Even in art supplies instead of paint store.

Get a small artist's brush to do touch-up with.

If you have splits in walls where the old & the new meet, maybe a special caulk could be used at least to seal the cracks. Make sure it is for walls & make sure it is paintable. Otherwise the advise for painting already given should suffice for the painting aspect of the question.

Have 2 or 3 different sizes on hand. I have a 2 inch and use it for all my cutting. A really good brand is Wooster, I am sure there are others.

put on painters tape, run a small thin bead of clear paintable caulk along the edge of painters tape....let the caulk dry and then paint and you will get a smooth line.

Thank you all! I'm on my third room so I've done everything mentioned and I live with it! I thought maybe there was some magic trick that I hadn't thought of. The pictures are of the first go- round.... Now is when I go back with the artist brush and obsess! But as someone said..... It's why I love it! I think....:)