I'm assuming you two met online and began a romance. Obviously, you didn't take into consideration the logistical cons of distance if the relationship worked long term. Or, your brain operates in a way that's less than the norm when it comes to being maternal. You'd be now making them pay the price for you choosing the most difficult form of dating there is--long distance.
You had choices to date locally, which wouldn't bring extreme stress to your children. And then after your grandma has done a major favor to you many years, you'd bring a large distance whereas it'd be harder to help her out as she's getting even older, and to not be able to see her great-grandkids very often with that distance? Plus, there father does see them once a week. That's not nothing.
As said, the right thing to do would be to do what you want only after the boys have flown the nest. You're the adult and should've used better reasoning when it came to dating.
I'll also add that teens are going through hormonal changes with puberty, making them more emotional, with a mix of very high high and very low lows if things aren't going well. Teen suicide is a thing. And especially for the 13 year old who could be at risk of bullying when moving to a new school, when kids can be at their meanest at that age. It happened to me when my family moved cross country when I was that age. I had a mental breakdown and seriously considered suicide for a time.
Children don't ask to be brought into the world. You're responsible for their well-being. Once they're independent and don't need your daily presence, that's your time to do as you please.