1.   Especially when the struggle is so difficult, and the behavior so enticing, pleasurable, and meaningful.

2.   He did not find the voyage itself so difficult or uncomfortable, so why was it that he felt so depressed?

3.   It must now face the imperial impotence that Britain has found it so difficult to adapt to.

4.   Once the motivation to stop arises, it is not so difficult to kick the habit.

5.   The terrain was so difficult that for part of the journey three locomotives were needed to pull one train.

6.   This does, however, prove so difficult in most cases that it is not recommended.

7.   This results from the fact that it is so difficult to distinguish deterministic chaos from highly random behavior.

8.   Those few days were so difficult that I Decided to leave my job.

9.   We are extremely fortunate that such a devastating virus is so difficult to effectively transmit from person to person.