Breaking up is a horrible thing and its effects can be pretty devastating. After 25 years of being with someone and probably thinking you know them well to find the whole world has changed for you and you are no longer with another is very very hard to come to terms with.
It is easy for your mother to say you need to move on, however actually doing so takes lots of things. It is not just willpower, but time and circumstance too.
The fact you are reminded of your loss by your ex dating someone you know does not help, it simply emphasizes what has happened.
How people do deal with such tragedies vary, I found under somewhat similar circumstances both using work as a means of occupying my mind, and making myself try to lead a full and social life where the two things that helped the most. It was slow.
Do you have anyone apart from you mum to help you, give support and understanding at the moment?
Sometimes people, even those who love, can simply fail to understand how another feels. Often it is the limitation of experience, if the event is outside their life them there is nothing in common. Sometimes it can be helpful to compare to some other event , a miscarriage, the death of a parent/child or other grief-ridden matter. Other times the gulf is simply too wide.
Do you think your mother does care but is stuck as to what to say? A I'm sure you would realize this not only makes what they say too little and maybe seeming heartless or unrealistic, but also discourages from them from venturing there in the fist place.
Some people have tried letters, setting out their difficulties and heart-break in detail, others have invited the other to attend their councilor or doctor so a third party can explain.
I will mention that Relationships Australia not only helps those in a relationship or separation but also those that are dealing with the aftermath. You might care to give them a ring in your state or territory. Talking to those with experience can greatly help at times.
You are not alone, and can talk here whenever you like