I tried to conceal this as much as possible, that I might not debar him from the pleasures natural to one who was entering on a new scene of life
A constant struggle for Victor is fighting to conceal the despair that he's putting himself through, trying not to damper other peoples moods.
But in Clerval I saw the image of my former self; he was inquisitive, and anxious to gain experience and instruction
Clerval is a rock for Frankenstein. A rock in the sense that he is keeping Victor grounded and reminding him why he's fighting the monster.
If this journey had taken place during my days of study and happiness, it would have afforded me inexpressible pleasure. But a blight had come over my existence
Mentions again how much of a happy person was before creating the monster. He also doesn't mention specifically what the "blight" is, not wanting to think about the monster more than he has to.