My daughter virtually lived with my Mum when she was young because I was studying at Uni, working 3 jobs and my DM would always get her started on something fun just before I arrived so DD was reluctant to go home. To be honest, I loved my daughter more than anything in the world but sometimes it was easier to take the path of least resistance so from afar it might have looked like I didn't put her first. When my DD was 4, my DM announced she was moving to Mexico for a year. My DD was devastated and sobbed loads and displayed similar signs of distress as your DGS. Frankly, I wondered how I would manage too but recognised my DM had to join my DSF. My DM wavered about going but I encouraged her to be with him. Within a week or two my daughter realised that the sky wasn't going to fall in and settled down. When my DM returned, things resumed as normal but I had finished Uni, was working full-time and my DD was at school, so I had more time so the time spent with my DM was somewhat reduced.
I am so glad my DM went though because my DSF died a year after they returned from Mexico and if she hadn't have gone, my DM wouldn't have had those wonderful memories with him, saved a baby who was fitting and received medical help for something the NHS had missed. Go danae give yourself a break, your DGS a chance to experience handling distance between you and enjoy your time when you get back to him.