I understand how you feel. Our elder DD emigrated with her husband and four kids to the USA several years ago and we have only seen them once since then. They visited at the beginning of 2020, for my DD's work, and we spent a very happy week with them. They were supposed to be coming back later that year but we were in lock down by then. They are keen for us to visit them and have offered to buy our tickets but my DH's health isn't good enough at the moment. And, to be honest, the price of insurance scares me, as we're old and have health issues.
However, modern technology does help. You don't even need to be formal about it, ie calling at a set time, etc. My DD and I chat to each other via video calls on Messenger, and usually a grandchild or two will drift by to say hello. It's not like being in the same room but it's a hell of a lot better than just voice calls and letters.
Don't let anyone make you feel guilty about crying and feeling sad. You're grieving. It's a big adventure for them but, for the people left behind, it feels like an empty hole in your life. People will tell you to be happy for them because they have moved for a better life, blah, blah, blah - as parents, we know that and we're happy that our kids are happy, but it doesn't stop us feeling bereft. If you want to cry, then cry.
I put on a brave face when I talked to my family, before and after they went, but in private I howled. I also ate peanut butter straight out of the jar. Somehow it helped.
It never stops hurting but you get used to it. It becomes bearable, although sometimes something will open up the wound. My family recently got American citizenship. They say it is just a formality, as they now have dual nationality, but it still hurt. My eldest DGD just got engaged. I don't know if we'll be able to go to her wedding and, if she has children, our DGGC will be far away.
These thoughts are painful but, most of the time, you learn to live with them.
The best advice I can give you is to cherish what is still here. Our younger DD lives near us with her three lovely children. We feel so lucky to have them in our lives practically every day. You have family who are still here. You will find that they are a huge source of comfort.