Reduction and Grammaticalization

I've recently been doing a lot of reading about how languages evolve
TAM, person marking, case markers etc... one of my presents for
Christmas was a book called "grammaticalization" which I asked for.
Typically I've thought of the "wearing down" of words as being almost
entirely due to regular sound changes but I'm beginning to wonder if
this is true, if sometimes changes can occur in the most often used
words and phrases which don't occur in the entire lexicon. Example:
gonna from going to... obviously the word boundary disappeared for them
to merge, but still /goInt@/ or something similar must for a start have
assimilated nt to get n, a sound change I'm not sure has happened in
recent english at least. In the book there are lots of examples where
the grammaticalized clitics, affixes undergo much more reduction than
the words they evolved from, and often this doesn't seem explainable in
terms of the sound changes of the language. So can a small number of
clitics etc and grammatical words simply be worn down and simplified by
constant use rather than more widespread regular sound changes?