Not so good—I know exactly how you feel。
不要说：我完全知道你的感受。

This is very tempting to say, but be careful: Even if you have experienced a loss, each person has their own unique path to travel so you can't know exactly how he feels.
我们总会说这句话，但是记住，即使你也失去过亲人，但每个人的生命旅途不一样，所以你并不可能完全知道别人的感受。

Better—I can't begin to understand how you feel.
不如说：我没法真正体会你现在的感受。