The ideal certificate type for your situation would be a wildcard certificate. This basically means you can secure *.companydomain.com. So, a wildcard cert would cover www.companydomain.com, secure.companydomain.com, webmail.companydomain.com and mail.companydomain.com with a single certificate purchase.
All that being said, depending on the company you purchased the original certificate from, if you contact them back you should be able to get a refund for the cert you purchased for secure.companyname.com and then purchase a wildcard. Most SSL companies I've dealt with are pretty forgiving - especially if you purchase a replacement cert - and a more expensive one at that - from them.