If you can re-make the sketch, it's better to sketch the centerline, then activate mirroring and draw the sketch "directly mirrored". In that way the program automatically resolves potential mistakes such as centerline-crossing (it will prompt you), and you can add dimensions and constraints in watever part of the sketch, no matter "left" or "right" (of course, the symmetry is implicit, so be careful not to over-constrain your sketch...)
Claudio