John,
To me every sin is serious -- what do you mean by "serious"?
Of course, all sin is serious. But some sins are different. I don't really have an exhaustive list at hand, but any sin that would canonically bar a person from communion without first going to confession first would be more serious. Keep in mind that like you said all sins are serious--I am not saying that one sin is greater than another--sin is poison in the veins, so to speak, no matter how little.
Most of us in the world are in such a spiritual state that sin is almost a constant companion. I'm hesitant to go into specifics because I am not ordained clergy, but to me: any sexual act outside of the confines of a Church blessed marital union, acts of violence towards yourself or another, missing church for an extended period of time for no good reason, etc. would all be sins I would consider of serious import, enough to seek confession before I approach the Chalice.
I myself have many sins which I heard one priest refer to as "dust", sins which aren't all consuming but still traps that I have fallen into--improper thoughts, sparks of anger and so forth--small, niggling things that cling to us and can build up over time. These still should be confessed but, assuming one has the blessing to confess only once a month or so--with no requirement to confess before each receiving of the Eucharist, I consider them to be serious but not urgent. Once again, this all depends on what your priest says and recommends for you personally. I am just a stranger online with absolutely no authority to dispensing spiritual direction.
As I wrote this reply, I was reminded of something I read once, that may or may not be applicable. I am not saying this is yours or anyone else's situation, but I found it useful--if you do, great, if not forget it.
"I knew a woman once, a spiritual child of Elder Sophrony’s, a middle-aged married woman with several children, who was overtaken suddenly by a painful psycho-spiritual illness: severe depression with suicidal thoughts, which took the form of religious mania. She was obsessed with forebodings of damnation and despair of forgiveness; made long catalogues of her minutest daily thoughts, no matter how fleeting, etc. In desperation, with her marriage almost over, she went to Essex and begged Fr. Sophrony for help. He told her to throw out all of her notebooks of sins, to read the Gospel of St. John every day for a year, to say the Jesus Prayer as much as she could [3], to receive Holy Communion as often as possible, and to come back to Essex for some time every year, to rest and pray there. She did as he said, and made slow progress at first; but after a few years she became free and whole again. She told me at first that she had to say the Prayer out loud as much as she could, because the minute she stopped, she began falling back into her “old crazy mind” as she called it; but little by little, she began having more time free of her fears. The Gospel of St. John, after many repetitions, forced her to see that God is really a God of love, who cares for her in a personal sense. This was reinforced by her practice of the Prayer and her visits with Fr. Sophrony. Over the course of time, she proved to have quite a gift of intercessory prayer for others and spent the remainder of her life, as her children were grown, living a quiet life, “only a housewife” to all appearances, but spending much time each day in prayer for others, a form of charity in which she was much aided in the great compassion for the sufferings of others that her own torment had given her."
http://orthodoxinfo.com/inquirers/safely-home-to-heaven.aspxOnce again, I am not saying you are like this woman, or anything like that. I merely found the anecdote helpful for me personally, when I found myself perhaps judging myself a little too harshly.
In any case, it all comes down to you, your priest, and God. If you find it best to commune only once a month, that's great. Frequency of communion will always vary, from case to case. Historically, different places followed different traditions in this area.
