Most of us struggle with sinful thoughts or desires of one nature or another every day. I have always been told that it is how we respond to these thoughts, not the fact that we have them, that can constitute sin, particularly mortal sin. In order for it to be a mortal sin it must be 1) grave or very serious in nature, 2) you must be aware that it is a grave sin, and 3) you must give full consent of your will to do it anyway. What you have described does not seem to meet that criteria. It seems to be more of a temptation that you are battling with. On occasion, you may find it more difficult to contend with and incur some sin. But unless you choose to pursue these thoughts after the realization that they are sinful, you are not committing a mortal sin.
This doesn’t mean you shouldn’t confess it though. By going to confession and asking forgiveness for venial sins, especially ones we find ourselves in regularly, we train ourselves to avoid that sin and receive God’s grace to help us do so.
And no, you should not feel like you have to wait until Saturday. If you feel you are in mortal sin, you should immediately make an Act of Contrition and proceed as soon as is reasonably possible to the sacrament. Please note I said reasonable. For example, I should think that unless you are in danger of death, it wouldn’t be reasonable to bang on the rectory door at 3 am or something, but at our parish it is perfectly acceptable to go to the office and ask if a priest is available for confession during normal office hours or to call for an appointment. In fact, it is recommended if your confession is anticipated to be very lengthy. I suppose this may be less available in parishes where priests are in shorter supply, but most priests take this need of the faithful very seriously and will make an effort to accomodate you.