You got the color pretty good but you used the wrong kind of stain. The mantle appears to have been done with an oil stain which brings out and enhances the grain. The sample was done with a latex or acrylic stain which subdues the grain.
At this point you might let the mantle dry for a week or so and then go over it with a paint and varnish remover and remove as much of the stain as you can. Let the remover soak a long time, the removers available today are a stretch to call them removers. The government banned the chemical that makes them effective. The next step would be to sand it some with some 180 grit paper. Then you could have your paint company mix some latex wall paint to a cocoa color and test a spot to see if the color is right. You probably will have to thin the paint to use as stain and wipe off the excess completely. Normally I suggest testing a finish on scraps but you would have to stain the scraps with the same stain you used on the mantle, let it dry and chemically strip it and sand it like the mantle. You could test directly on the mantle but have a rag and some lacquer thinner handy in case you need to wash off the stain.