There are opportunities that a suicide can do teshuva (not all methods of suicide produce instantaneous death) and yet we still say that they have no chelek in olam habah.
Those type of suicdes, where he may have done teshuva, such as if he sufficates himself with a bag – we in fact do assume he did teshuva (ie after he could no longer remove the bag but was still concious enough to do teshuva). And in such a suicide we dont bury him in the outskirts of the cemetary, as we would if it were an instant suicide.