What can I do about the large gap between my new 6-tooth bridge and my gum? Air whistles through, I can't say "s" properly.

Had both upper front teeth removed, long standing infection. Immediately had a temp 2-tooth denture added, then 4 side teeth cut back, then 6-tooth bridge cemented in. Now: small gap visible at front, large gap at back which I can feel with my tongue. Dentist said: bone above 2 front teeth has shrunk (since the teeth were removed, after impression was made), therefore there is this gap. Does this sound correct? I can live with the gap, but speech is dramatically affected. Help.