If you’re (general) involved with someone else and saying this to your ex, you are 100% having an emotional affair and need to end the current relationship you are in.
Is this you or your so doing this? If it's you, how would you feel if it was your so telling another woman he loves her? Would you be pissed? Then yes, it is cheating. If this is him doing it...well, same as above.
Absolutely. You should never be with someone if you still love someone else. It's also way out of line to actually go express your love to that person also.