Sorry this is happening. Unfortunately you seem to have extremely strict oppressive parents.
There's a few things going on here. One is she is away at university and enjoying her freedom and youth and campus life, as she should. A lot of highschool romances drift apart because of that phenomenon.
The other is that she represents the rebellion you are terrified of. You are so utterly afraid of and controlled by your mother, that being with her was a sort of passive aggressive way to combat that.
Please get the best grades you can and talk to highschool mentors, teachers and guidance counselors about scholarships, financial aid and going to university as far away as possible from your parents.
Please also talk to other adults about your parents behavior. Perhaps you could use some deprogramming from years of this toxic umbilical cord.
As far as this highschool GF, leave her alone. Set each other free to grow and live life.