Sounds like she wants what she wants and frankly she is being very selfish. If you really love someone you want the best for him/her always. Sounds like she wants you to do what she wants and if you deviate from that then there will be issues/problems/tantrums=red flag.
Do what you think is best; follow your gut.
Sounds like this relationship will not really survive in my opinion WITHOUT serious conflict. Plus the odds are already against you being you've already broke up once (red flag) and you two were "high school sweethearts" (red flag as these types of relationships have a propensity to not last life long).
This opportunity with all the benefits they're offering you sounds amazing. Not many people get offered such a generous path to starting off their career. Very few people, in fact. With the state of the economy right now with no relief in sight, you would be insane not to take this opportunity to get ahead in your future. You need to take this job. Literally your future could depend on it.
As for your girlfriend, honestly it sounds to me like she is jealous. You've been offered a sweet deal and she still has to find something to do. I'm sure part of it is jealousy. But another thing you should keep in mind is that, if she gets this weird over something that will absolutely improve your future, then you need to ask yourself if she is worth it. Cause a smart girl would see anything benefiting you careerwise as positive, if the plan is for you guys to get married eventually. Since she is too jealous to see this, I think you need to ask yourself if this is someone worth anymore of your time.
Ugh, sorry to hear this. She does seem to be acting as a tyrant having a tantrum. Now, I get that you two had a plan and maybe you've changed the dynamics. And she wants a job after college as well and the idea of the area you have to live may afford her less opportunity. Is that partly why she is upset? And if she wants to get her masters, then let her get her masters. I can't imagine a boyfriend, even a serious one, dictating to me my education level, career, etc. It's not right for her to do that to you or you to do that to her.
Many a young love falls apart when 'real' life begins. Take care of the structure of your life (education and job) and if she is meant to be, she'll work through her issues. If not, then you WILL find a partner that works better through the hard times.
If she had tried to talk to you and told you that she is unhappy with your decision (had you talked to her while making it at all to get input?) that is one thing. But treating you badly isn't a mature person handling stress and hence, not a great life partner. good luck
I agree with Your Family and Your Gut
sometimes "love" isn't enough
This is a major issue and You must have agreement. If either of You sacrifice this issue for the other, the relationship probably will not survive.
Good Luck With Your Decision
("hostility" is a very different reaction from 'disappointed' or 'reluctant'. Don't overlook Your choice of word here - "hostility". This is probably a Red Flag)