You may want to consider reapplying to the two Ivies that you turned down for Amherst. They obviously saw your strengths when you were a high school student. I also think many universities would be sympathetic to an otherwise excellent student whose grades fell off due to freshman adjustment issues. You will need to give good reasons for why you think these schools will now be the right fit for you, how you would contribute, and what you hope to gain from attending X university.
Many state universities have Honors Colleges, and those students get smaller, more high-powered courses, interactions with academically strong classmates, and better access to faculty, advisors, and alumni. Some students at our state university Honors College have turned down Ivy league schools and places like Amherst. For middle to upper middle class families, it is an attractive option without the whole family going into debt.
Last, you have you maximised your experience at Amherst? As suggested by others here, joining different clubs, activities, programs, etc will link you with other people. There also are some 5 College organizations that you could participate in that may also broaden your horizons. To me, Amherst never seemed too small. I always was meeting new people, even from my own class, as well as getting to know students from other colleges.