Yesterday I sent a Happy New Year to my girlfriend who broke up with me and I still haven’t received a reply yet?

This should not be blamed on the ex-girlfriend. Now that they have broken up, why bother trying to tease her nerves.

For now you should wait quietly.

If she replies with a blessing, it can only be regarded as a courtesy greeting.

If the other party does not reply, then you should also be calm. What you and she need to face is now, not the past.