I used to have the problem of speaking too fast as well as dropping a lot of sounds (s, z, l, f, and v's), but after my school dumped their poor excuse of a speech therapist (made me read poorly written poems and interrupted me every time I made a mistake) and replaced her with a new one who focused more on (proper articulation and self monitoring) my speed slowed down while my volume and general intelligibility went up.
This may be just me (I have a lot of missing muscle and nerve tissue from a bout of neuroblastoma in the neck) but try focusing on feeling and slightly exagerating your sounds.
