Hi Depo. Because of the joking nature of the letter it doesn't need to be strictly formally correct, but I would still write out single digit numbers like "7" as "seven". Either is OK in this context.
I assume you mean the movie title The Wolfman rather than the character himself, so I would italicize it, or put it in quotes, "The Wolfman". With Snow White and the others you are referring to the characters, so they are OK. If you mean the character then it's Wolfman.
I wasn't sure whether you rejected the online proposal because she said she still wouldn't marry you until five years had past. If so, "cannot wait" or "could not wait" are both OK. The "for" following "wait" is optional.
The "P.S." should come after the sign-off.
I’m so sorry I haven’t been paying any attention to you lately. It wasn't to do with anything you did or said, it’s just that I was busy with Snow White and Sleeping Beauty. I thought I’d found true love in Snow White, but then found out that she’d cheated on me with seven other men (midgets actually). Then I was caught up in a fling with Sleeping Beauty and discovered the actual reason why they call her ‘sleeping’ beauty. Anyway, it made me realize that you are the only true love in my life and I’m truly, madly and deeply in love with you. I know I rejected your proposal online but I simply cannot wait for another five years to marry you. So, my dear sweet Cinderella, will you marry me? (I’m serious!!)
P.S. We can get married in a theatre while watching The Wolfman.