you can use familysearches free PAF geneological program. you can put your ged into it then press download and select parcial. on the next screen you can point to yor mother and select from the dropdown tab which rellies to download eg all ancestral related so filtering out those you dont want on the resulting tab. the next thing to happen is that you can see which will and wont be used in your new created ged then all you need do once happy is push download. and your original ged will still be there and the new one from your mother will be in a new file.

sounds complicated but it is very easy to do as you are guied step by step.

good luck