Colleges recruit kids based on how good they are and how good they project them to be. If you were not offered, they do not see you developing into a player who can start on their team. A postgrad year of high school won't do much to change that. However if you end up at a prestigious boarding school as a postgrad it can help in admissions as they will most likely see an older, more mature student, capable of living away from home.
Baseball can still be an option for you but you will have to look at D3s or Patriot League. Unclear if you want to do that, but if it came down to attending an Ivy as a regular student or playing ball at a D3 you would get a mixed pot of answers. The good news is the student body probably won't be loaded with OFs throwing 91 so you chances of walking on are better than most as long as you can hit with authority. There isn't any scholarship money invested in the recruited athletes so if there is a walk on that is better than a recruited player, they will keep you in the mix.