I had a similar situation. I came out to my mother and step-father the beginning of my senior year of high school. They really are exceptionally progressive people, so I knew that there would be no problems. In fact, my mom has always said "I don't care who you bring home - it can be a guy or girl - you just better tell me so I can cook enough for another person at dinner."
I had come out to a few choice close friends, but other than that, I was still in the closet.
I finally graduated and came to college - and decided that it was time.
I drove home to come out to my father and grandparents, who did not know (and apparently had no idea).
It wasn't as miserable as I thought it was going to be, but my father wasn't exactly accepting.
Regardless of how ready I was, it was still hard to come out. It always will be.
From my personal experiences, I would recommend suffering these next few months until you can graduate high school and move to college - that's what I did, and it worked wonders. I totally agree with KaraBulut with the "exhale" thing. It's so true.
College allows you to meet people who are accepting of you. Sure, there are those who aren't, but there are so many people at universities that if someone is an asshole, you can just ignore them and find someone new.
Just stay strong and be safe.