I really think that your adult children would expect you to visit and be part of their lives, this is normal, they wouldn't not want to see you just because they are married.
We moved to another country many years ago and when my husband's mother wants to come, she doesn't ask, we expect her to want to come, and she just books her flight and tells us she is on her way, and that is fantastic, she is very welcome. The same with other relatives.
If my mother and father were still alive, I would expect them also to just come and see us, as they are family.
Obviously I do not know your circumstances, but if you have lost contact, then why not go on Skype or Facebook and start to get involved again, and pick up the relationship where it left off, and then when you feel a little more comfortable speak to your children and tell them that you would love to see them. You are their mother, you should see them, wherever they have moved to, and no matter how their circumstances have changed.
You should be a part of their lives, definitely, a big part, you are family, and this is absolutely right, so don't feel like an outsider, you were the most important person in their lives once, and you are still important.
You should feel that you can ask your children openly and not feel like an outsider, and you must communicate how you feel so that you can understand where you fit in with these changed circumstances.