Today I didn’t take the eucharist because I didn’t know if the sins I comitted were mortal (I missed confession). So how could I knew whether I have or not?
Ask the priest while in Confession. Your sins could be forgiven then either way!
Missing confession in it of itself is not a sin unless you fil to perform your Easter duty.
were any of your sins grave matter (serious enough to cut yourself off from God and the Church), did you know they were grave, and did you deliberately choose to do them anyway?
Well, they are not really horrible I just really wasn’t sure if they were mortal and I had some questions to ask. Except when I got their the lights in the confession room were off and there was no priest 
.
As noted above to commit a mortal sin three conditions are necessary. 1. It must be a grave matter (you read or someone taught you that was. Maybe it was something like murder where the gravity is obvious.) 2. You most know it is a grave matter or think that it is a grave matter. and 3. despite all this knowledge you decide to do it anyway. As a priest once advised me if you commit a mortal sin you will know it . If you don’t know it than you could hardly be culpable. If you hang up on the freely decided part you really need a priest to evaluate the level of culpability in confession as there are conditions which diminish culpability or guilt. 
As a priest once advised me if you commit a mortal sin you will know it . If you don’t know it than you could hardly be culpable.
This doesn’t sound right to me. The CCC says
"1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent…
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin."
The way I read this, for a sin to be mortal you must be aware that you are sinning, but you don’t necessarily need to be aware of the gravity of the sin.
For example, if a Catholic knows that missing Mass is wrong but misses anyway, they just committed a mortal sin even though they might not have realized that missing Mass is a grave matter. However a Catholic who converts to Protestantism honestly doesn’t think missing Mass is a sin at all and so they would not have committed a mortal sin by missing.
Here’s the thread in Q&A from which I got the last paragraph
If you don’t know, you probably didn’t. Mortal sin is a deliberate rebellion against God. Read this: Ten Commandments for the Scrupulous
Scott
I’ve found that it is not useful to think in terms of mortal and venial sin, and in fact this isn’t a division used in 23 out of 24 Catholic Churches. To the best of my knowledge it is only a theological distinction the Roman church makes.
If you are motivated to go to go to Mass and confession then I find it very hard to understand how a mortal sin has occurred. A mortal sin cuts you off from divine life. It is the kind of sin that causes one to leave the Church, and by all means if your sin has caused you to stop participating in the community life of the Church then you need to go to confession.
Although of course this isn’t the only reason you should go to confession. The point of confession outside of just restoring one to a state of grace is that all sin no matter how private is actually something that damages the whole community. We confess to our bishop–or a priest he has given such authority–in part to acknowledge the damage our sin has caused to our community, and not because God won’t forgive us otherwise.
The best practice I have found is to give up thinking about your sin in mortal or venial terms and simply go to regular confession along with regular communion. Confessing all of your sins without distinction in confession–that is to the best of your ability. Obviously you should remember your most serious sins by their number of occurrences, which you are able to do because you are also doing a daily examination of conscious.
If that stuff isn’t being done, then you have to wonder if the sacrament is really even valid anyway?
Also remember the three pillars of a Christian’s life: prayer, fasting and almsgiving.
We concern ourselves with questions like these meanwhile the Church has reduced the number of proscribed fast day to only two. I think these two things are most certainly related.
Really you should be fasting 46% percent of the year (every Wen.& Fri that isn’t a feast, Advent, Great lent, apostle’s lent, and assumption lent) and feasting 33% of the year (every Sun. Christmastide, Eastertide, apostles’ feast, assumption feast and all other proscribed feasts throughout the year)
In my opinion our time and energy is better spent organizing our live’s around fasts and feasts than worrying which sins are venial and which might be mortal–because if you ever fall into a state sinning too such an extent that the fast, feast, prayer and almgiving order of your life becomes ruptured (which is basically the state of the whole Roman Church–at least in the U.S.) then you’ll know you need to get to confession or your very salvation might be in jeopardy.
If you are unsure of whether or not your sins are mortal and your conscience tells you that you ought not receive Communion, then you should stay in your pew and make a spiritual Communion instead. You did the right thing.
There is a HUGE difference between mortal and venial sin. A mortal sin cuts you off from Christ and death in a state of mortal sin will result in eternal damnation.
Mortal sins are not simply sins such as murder, rape and armed robbery. Grave matter is a breach of the 10 Commandments, and if that is done with full knowledge and deliberate consent then the sin is mortal.
Yes I agree that we should confess all our sins, both mortal and venial in Confession. All sin is serious. But to suggest that it doesn’t matter if a person receives Communion in a state of what could be unconfessed mortal sin is very dangerous advice to give (particularly when the person’s conscience effectively urged him to remain in the pews).
If in doubt, stay in the pews and get to Confession as soon as possible.