Everything you disclosed in your original application retains its priority date of the initial filing date. The new material you added to the CIP gets the date of the filing of the CIP.
If the other inventor's application was filed in between those two, then your initial filing can be used as prior art against the other inventor's application, and the other application can be used as prior art against the new matter in your CIP.
"First to invent" is a bit more slippery than that, though. You may still be able to prove earlier invention through showing that you had invented the subject matter earlier, e.g., through witnessed lab notebooks, and that you were diligent in pursuing the invention and the patent applications.
Of course, the OTHER inventor can also try to prove the same thing, and may be able to pre-date your original filing.
It sounds like you might be headed for an interference hearing before the BPAI. You would be very well advised to find a patent attorney to help you, if you don't have one already.