The pain is less now than it was 5 hours ago
Shouldn't it be "The pain is lesser now than it was 5 hours ago"?
Based on Shoe's answer (the most upvoted answer) lesser can only be used attributively. So you can't say:
The pain is lesser. You can only say a lesser pain.
Consequently the correct use would be:
The pain is less.
While "the pain is lesser" is wrong. "The pain is less" sounds very clumsy to me at best and might actually also be ungrammatical.
You could say "I'm in less pain," or "There is less pain now"
If you insist on using the construction you started with I would recommend
The pain is smaller now than it was 5 hours ago."
I think the reason "The pain is less" sounds so strange is that you would really expect this use of less to also be modifier. For example "The pain is less severe now then it was 5 hours ago" works fine.