When a catholic eats the eucharist, what eternal effects does it have on them?

Does it have a lasting, eternal effect? Or does the effect of it, whatever the effect may be, does it wear off after a while? The reason Im asking is because I know that before a catholic goes to partake of the eucharist they say, "Lord I am not worthy to receive you....." and that worries me, because they ate the eucharist last Sunday...so obviously, whatever effect it has on that day ( last Sunday), it wore off. So what lasting , etenal effect does it have, it any at all? Surely there must be some eternal lasting effect. Help me out here. I was told that the effects of the eucharsit can be destroyed by sin...as in when the catholic who ate it sins...then that destroys the effects of the eucharist..so it seems that sin is more powerful than even the eucharist, if thats the case.