I'm afraid this backup has overwritten the old one. You can check if it's still available by going to the Google Drive app, then Menu (3 horizontal bars) > Backups.
If you can find your old backup there, delete the new one and go back to WhatsApp to restore your backup containing your chats.
Otherwise, try looking for a local backup. From your phone's file explorer, find the WhatsApp folder and go to Databases. If by chance there are files named "msgstore-YYYY-MM-DD.1.db.crypt12", rename the backup to restore to "msgstore.db.crypt12" and reinstall WhatsApp to restore it.