Hi there SimonsKat-
I understand your feelings. I have had several long distance vanilla relationships and they were very hard on me. I often felt like I should just pick up and go wherever they were. How long have you guys been together? Do you guys have any plans for the future? I think it is important to know each other's expectations, especially if you have been together for a long time.
Personally, I have never had a BDSM relationship 24/7 before- until recently I just met a new Master. He lives in the Canada and I live in the US- so distance is a problem. Certainly not as big of a problem as it is for you- but even still I feel the sting of not being able to get together in person. I can only hope that if things work out between us-somehow we will end up near each other- that is why I ask if you guys had any such plans between you. At least if there was a plan for you of being together in the future, it may help you handle the sadness of being apart for so long. Give you something to look forward to.
Try to keep communication frequent, and it is okay for you to express your feelings to your Master. I am sure he misses you as well. Maybe making friends in similar situations and talking about it with them would help you. Sometimes I think there can be added loneliness if your Master is the only one that knows about your kinks.
Anyway- I am not sure if this has been helpful. I guess I'd like to know more information in order to give some better advice. In any case, I sympathize with you, and hope things improve for you dear.