As far as I know, the only sin spoken of in scripture as
"unforgiveable" is blasphemy against the Holy Spirit. Jesus is
recorded as saying this in three of the four gospels: Matthew
12:31,32; Mark 3:28-30; Luke 12:10. (To me, the most troubling
aspect of blasphemy, or "evil speaking", is defining it. Biblical
examples seem to run the spectrum from casually irreverent speech or
behavior to defiant railing against God and ascribing His work to
Satan.)
Regarding suicide, I suspect that "inability" to ask forgiveness for a
sin is more tolerable by God than "neglecting" to ask forgiveness -
which we have all already done countless times. Hopefully, He
forgives us in spite of our occasional neglect to ask or else we are
all in deep trouble.
Wayne