Tbh I don’t think there’s much hope of this relationship working on either side.
It’s very easy to suggest that the bloke is in the wrong, that he’s accepted a job without considering the OP, but that works both ways.
OP currently lives with her grandmother who she is caring for? So how would that work if she and her partner wanted to move in together? Get married? Have children? Reality is that it wouldn’t, and truth here is that currently it is the DP having to make all the sacrifices, do all the travelling for him and OP to see each other, he would have to be the one who moved and then moved in with the OP’s disabled grandmother.
That’s not a solid basis for a relationship either, and I do think that the OP needs to consider her options with regard to her caring responsibilities and where those responsibilities place her with regards to having relationships in the future. Because if it’s not this bloke, it will be someone else, and the OP’s circumstances are such that they will affect her relationships in the future. And expecting her to be a full-time carer at 26 and for the foreseeable future is very unreasonable.
And with regards to the DP, OP said he started the application process quite a long time ago, so presumably when the relationship was far less established than it is now. And having not been able to find the same work here, he is not unreasonable to turn down the offer in favour of moving in with the OP and her grandmother, with no future job prospects for him here.
I would consider this relationship to be over, but through circumstance not through anyone’s fault. Right person perhaps, wrong time. If it’s meant to be, then in three years time you may both be in the right place, but as things currently stand you’re not.