You need to realize this is NOT your landlord's problem..it is YOUR problem. The landlord is free to pursue the rent from both or either of you - that is what jointly/severally liable means. There is no "her share" vs "my share". At least not to the LL. Right now the rent is underpaid, there is a balance outstanding and you both will be the defendents on an unlawful detainer lawsuit if it is not paid. I explained the consequences of that above.

You should not enter into legally binding contracts with people you really don't know/trust. Pay the balance of the rent and keep pursuing the roommate on your own, not bothering the LL about it.

The landlord MAY be open to letting you out of your lease if in the end the roommate does really move out for good and you can't afford the place BUT the landlord does NOT HAVE TO DO THIS. Also..you cannot just move in someone new..the landlord has the right to fully screen and approve whoever you wish to move in and only IF the landlord approves the new roommate can they move in.