Sorry to hear you have lost your conversation, backup your data as frequently as possible to avoid this in the future. You may however have some options for recovering.
You could attempt to recover the conversation from any of your other Apple devices if you were using iMessages on any of your devices other than your phone. Take a look here to see how you can transfer messages between devices.
Lastly you could have your mother forward over the entire conversation from her end for you to keep in an email, Google Doc, or Office 365 Doc for future reference. Click or tap here to see how she can do this.
I hope this helps you, come back and let us know if either of these solutions worked!