If you had asked the teacher for explanation,you____difficulty now.

应该是wouldn't have
因为wouldn't have had是过去式
句子是“你现在就不会觉得难”所以应该用现在式的wouldn't have

wouldn't have had
虚拟语气

表达的意思是
如果你早让老师给解释，你现在本不应该有困难了
(结果是你没有让老师给解释，所以你有困难了）

虚拟语气的构成就是
与过去事实相反
从句had+ -ED分词
主句would(第一人称可用should)
+have + -ED分词

与将来事实相反
从句were to + 动词原形
主句would(第一人称可用should)
+动词原形

而此句是与过去事实相反（ had asked ）所以用wouldn't have had

语法上 ，两个都对
意思也一样
你觉得那个顺口 ，就用吧

应该用wouldn't have had，wouldn't have done表示一种猜测的语气，表示不可能怎么怎么样。