Have you discussed with your cotenant that you want to move out?
Regardless of the circumstances you can’t just break the lease if its a fixes term lease.
If your cotenant is agreeable to taking over the whole lease, then she can pay you your share of the bond. But she may noy pay you the whole amount if she thinks the agent will not pay back the whole amount of the bond at the end of the lease.
If your cotenant is not willing to take on the whole lease, then you can notify the agent that you both want to break the lease. There is a fixed notice period. Your lease will also tell you what the break fee is. When you both move out the agent will inspect the property and return your bond, less your break fee and less anything they charge for property damage.
Alternatively you could find another tenant to replace you. Until you find a new tenant, you are liable for your share of fhe rent. You wwould then negotiate with the new tenant to pay you your share of the bond.
Its sad and uncomfortable when cotenancies go pear shaped. You both entered into a contract and are responsible for it separately and jointly.
Its going to cost you some money to get out of the lease. Just accept that fact. Act honestly and you’ll get out of it at least cost.