Your deal with the storage co is your deal. You are liable till the end of the lease. Give the ex written notice that she has to remove her stuff or you will throw it out/donate it etc.
Answer Applies to: New York
Tell her that you consider that she has abandoned her stuff by not coming and picking it up. if she does not come and get it all .. sell it or abandon it. Talk to the owner/manager of the storage place and see if they will auction it off for unpaid fees if you stop paying.
Answer Applies to: Michigan
I would send her a letter with the keys telling her that you have paid one (more) month's rent and after March 31, 2014 the obligation to pay rent on the place is hers and if she doesn't do it, she may lose her possessions when the contents are sold.
Answer Applies to: Idaho
Legal technicalities aside, the simplest approach might be to ship everything to the x-girl friend. Legally, you could send her a 30 day notice of terms of the storage rental, stating that "Commencing 35 days after the date/mailing of the notice, there would be a monthly rent charge of $200.00, and that the failure to pay the rent could result in the contents of the storage unit being sold at a public auction, with proceeds being used to pay past due rent, and the balance being forwarded to the owner." It is not quite as simple as stated, but that is a shorthand summary of the actual procedure.
Answer Applies to: California