If you truly come up empty when you examine yourself
for sins (whether they seem mortal or not), and are
truly at a loss for a sin, that would explain
this; don't worry.
We base our faith not on what we feel, but on objective
facts. If your conscience is totally clear, there
is no reason to believe you are not worthy to receive
Communion. If you have some sin that you're not sure
about, that you haven't confessed, then try repenting
of and confessing that, but it is possible you're
just in a strange psychological state.
You don't have to reply to this, but consider
whether you feel unworthy in other aspects of your
life. If so, you may be depressed or experiencing
anxiety and a trip to a therapist or psychiatrist
may be in order.
You may want to talk to your local
priest; he can better explore this with you.
Hope this helps,