napisałabym to tak:
Dear Louis, /bez wykrzyknienia, ale z przecinkiem/
I know you haven't heard from me for a long time and I'm really sorry.
I`m writing to let you know that I won`t be able to spend this
weekend with you so we can`t go skiing together.
I can't come because I've got an important exam in Maths (Math - amer.ang. - nie wiem, jaką wersję angielskiego preferuje nauczyciel;) and I mustn`t miss it.
Anyway, I`d be pleased to come at another time if it`s allright with you:
I could change the reservation in the hostel but I would need to do this soon.
Or I guess it would also be fine if you took someone else to spend the weekend with you.
I am waiting anxiously for your reply.
Take care,
vvv
YOU - nie piszemy z dużej litery (wiem, że tak jest grzecznie w j.polskim, ale w ang. to nie ma znaczenia i może po prostu nie zostać zrozumiane).
Pozdrowienia!:)