You said you cut the old solder type valves out. Did you cut back past all the old solder? If you did you might have gotten into the bend area of the pipe which might be a bit out-of-round. Don’t cut the pipe with a saw. Use a wheel cutter. That will both keep the pipe roundness and/or correct a bit of ovalness.
It’s OK to seat the new compression fitting on the old solder area but it’s usually necessary to heat, then wipe the area, to get rid of excess/unequal solder. And know your actual tubing diameter. Make sure, by measuring, if it’s uniform. Older pipe might be a bit oversize compared to current production. Make sure your new compression nut and ferrule are proper and matching. You said you had trouble getting the new nut on. Same with the new ferrule?.
Welcome to plumbing 101.
I’m past the solder. The pipe is pretty far out of the wall, but it does have a nasty curve to it. If it’s oversized relative to new, is there a way around that? Do companies sell slightly larger than 1/2” valves?