You are not being selfish.. Your not going to like this bit, I say it with respect but it is your Mum that is being selfish, maybe out of desperation and panic.
Nothing will change, you will still be seeing her every weekend so where is the problem
Being alone isn't all its cracked up go be, especially if you have been with this man for so long
This bit of my in put is a bit personal but if you have stayed together, even though, for the last ten years you haven't been close psychically says alot, there must be something left, whether it be respect or deep real love.
I had trouble with my marriage when my he was still alive and all I kept thinking about was running for the door, getting my own place, starting a fresh. But..looking back now, and if I had my chance again I would work harder make more effort with my husband.
Going from one relationship to a different or new relationship is really just swapping one lot of problems for a different type of problems, because no one is perfect.
Spend some time thinking things through in peace and with an open mind and an open heart, before you make you final decision and move out.
I wish someone had said this to me whilst my husband was still alive..