You need to convince your mortgage provider that your name can be removed. At present the mortgage provider has two people named on the mortgage document both of whom are liable for the entire payment (not 50%) your difficulty may be convincing them to reduce this.
The other thing you can do is to cease paying the mortgage, the provider may then recover the property and have it sold, or your ex-partner must pay the full amount.
You may find it difficult to buy another property whilst named on a mortgage document or having had a property recovered through non-payment.