You’re welcome.
If by fantasize, you mean deliberately initiate the thoughts and willfully and pleasurably entertain them, then that may be a mortal sin. But I really don’t know. This subject should be discussed face to face between you and your confessor.
Above I wasn’t referring to fantasizing, but more to the arising of a non-deliberate thought. For instance, if someone ran into my car, I might get angry and want to hurt them. These thoughts and feelings may be more than fleeting, but if I am not deliberately entertaining and nurturing them, they are a venial sin, not a mortal one. This is because they arise naturally out of our own personalities and spiritual state. I think that a person can work towards a relationship with God where these thoughts and feelings would be less likely to arise, or would be more fleeting when they do.
The teaching of the Church is that participation in the Sacraments, especially the Eucharist removes the guilt of venial sin. These sins do not have to be confessed, but they don’t have to be withheld in confession either. If you have any doubt about whether a sin is venial or not, confess it.
Also, you do not have to forego communion if confession is not available to you. You have to be willing and ready to confess, truly contrite and go to confession at the next opportunity. Don’t miss communion! It is Christ Himself and He will heal you and give you the grace you need to get over this problem and not sin.
Personally, while I do think that it’s something of a scandal that more Catholics don’t go to confession, I take advantage of the fact that there isn’t a long line. I’m a graduate student and so busy that if my head weren’t attached I’d lose it. So, when I go to confession, it’s probably the only time I’ll see my confessor other than Mass. So I talk to him instead of just confess my sins. I ask about these exact kinds of things. Doing this has greatly helped my understanding and put me at peace with my relationship to God.
If your confessor is not someone who you think you can do this with, find a different confessor. Either a different priest at your parish or go to a neighboring parish. Personally, I think that the more conservative a priest, the better the advice I get in confession. More liberal priests seem to sometimes be dismissive of the seriousness of sin.