The Mesilat Yesharim (ch.4) quotes the Talmud (Bava Batra 75b) which says in Olam Haba (the World to Come) every person will be singed by the chupah (canopy) of his fellow, an analogy referring to the regret one feels for not attaining what his fellow had attained and that for those who were slack "there is no doubt, that their suffering will be enormous and everlasting".
Why is Olam Haba a reward then if everyone except for the top people (who fulfilled their full potential) are just going to suffer forever.