fear to do表示害怕做“某一件”具体的事；fear doing 表示一贯性的害怕做某事.举两个例子给你比较：
I fear to ask the boss for a raise now,because he's in a bad mood today.我不敢现在去找老板要求加薪,他今天心情不好.
I fear speaking in the public.I'm so shy.我害怕当众讲话,我太害羞呢.