In your "wrong" sentence, "I'm sure of it" is functioning similar to other filler phrases (for lack of a better term) in the spirit of "you can bet", "for sure", "certainly", "of course", and others. As far as I know, all of those can be inserted pretty much wherever it makes sense, thus justifying your example (although repeating with "it" is awkward). There are many great ways to write this sentence, and here are some I could come up with.
I think this year will be a good year; in fact, I'm sure of it.
I'm sure this year will be a good year.
I think this year will be a good year, I'm sure
Surely, I think, this year will be a good one (replacing "a good year" with "a good one" is relaxed but perfectly fine).
This year will be a good one, I'm sure of it!
Your example c) is awkward; it doesn't mke sense to use "and" there to compound on your "I think" with "I'm sure". "And" can be used there for different things "I think [...] and I'm sure it will be better than last year," but it's repetitive here to conjoin like that.