I am not an expert of SAT. As a reader, I just have some ideas.
However, sometimes we are more likely to make right decisions if we stick to our own ideas.
This sentence, quoted from the first paragraph, is your main idea.
Your main idea is fine. I believe many would agree with you. However, you did not give any reasons why you advocate this statement in your essay.
Indeed, you gave two examples, which are Garrison's and Ford's deeds. They are great men, making great decisions. But these examples are not reasons. In my opinion, examples support reasons. Lacking of reasons, your essay is not persuasive enough even with two good examples.
I suggest you should provide some reasons, such as...
Reason 1: After meticulous consideration, one should feel confident of his or her decisions. Because no one can know the situation better than he or she does.
Reason 2: To succeed, people should be persistent in the path they choose.
Then you can offer some examples to buttress reason 1 and 2.
I am not a good writer, so my reasons may be bad. However, my point is you should develop reasons before talking about your examples.
Because you did not use reasons to develop your main idea, your argument is unfounded.
In the examples above, both Ford and Garrison believed their egalitarian views as morally justified. Thus, we can conclude that it is best for people not to change their opinions they believe as right.
These are your ending sentences. Because of Ford's and Garrison's success, people should not change their minds when they think they are correct? How could this be true? For example, Hitler believed what he did were 100% correct. Unfortunately, we all know what happened.