Your in a bit of a pickle if your roommate does not want to cooperate. You can vacate the property yourself, however, you are on the lease and if you leave, you and your roommate both would still be liable for the rent, and the condition of the property, and if the rent is not paid, then the landlord may possible start the eviction process against you both, which will reflect on your credit report. You may be liable to pay the lease out up to it's term (ie: one year).
Look at the term on your lease. Are you on a month to month tenancy now? OR still within your lease term? Talk to your landlord. Also call the City of Santa Monica Rent Control and see what they advise.
And as always, speak with an attorney who may better advise you.
You can call the Coalition for Economic Survival at 1 (213) 252-4411 which has workshops for tenants with issues. They work by donation.
Kat Becker, Realtor
Prudential California Realty
Why didn't your new roommate want to be put on the lease? The rule tends to be that everyone that is living in the unit has to be on the lease and your lease probably doesn't allow sub-lessees (you're the landlord to your roommate).