When praying the Litany of Our Lady the end prayer changes for the season. Now some of those prayers have .... at the end meaning I have to add a collect after to complete it. I just realised that sometimes I have said the wrong collect causing me to address Mary as God.

For example,
"Jesus Christ who liveth and reigneth with Thee, and the Holy Spirit, God, world without end."

Is this a mortal sin? And how do you know which collects to use?

If it was a mistake, it would definitely not be a mortal sin.

To be mortal a sin must:
1.  Be of serious matter
2.  You must know that it is wrong
3.  You must give full consent to it.

As for which collects to use...  I rarely get to use my missal any more because of the little boys.  So, maybe someone else could advise you there.  

You seem to have a serious issue with scruples.  No one can accidentally commit a mortal sin, where you lose sanctifying grace.  That’s Moral Theology 101.

Now, one could accidentally do  something that is grave matter.  So, for instance, I see a $100 bill on a table.  Mistakenly thinking it’s mine, I take it.  But it’s actually someone else’s.  Once you realize your mistake, you’re required to make restitution.

Your mistake above, however, is not something that requires restitution other than simply correcting the mistake going forward.

Grave matter is different from mortal sin, which destroys sanctifying grace.  One can never accidentally lose the grace of God and commit a mortal sin.  For mortal sin, you have to know that it’s grave matter, realize what you’re doing while you’re doing it, and intend to do it anyway.

Would this be what's called a "material sin"?  If so, I'd be interested in learning more about that.

I've never heard the term "material sin".  Sin by its definition typically involves having met all the conditions for sin.

Term that's generally used is "grave matter".  This means that whatever has happened is serious enough where, if someone did it knowingly and intentionally, it would constitute mortal sin.

Bottom line is that you can't accidentally commit a mortal sin that would deprive your soul of sanctifying grace.

Anything spoken by complete accident is no sin at all. In this case, if you were in Our Lord’s home and accidentally addressed His Mother as Lord, I’m sure both would know it was a stumble of the tongue. Better to give more honor than to diminish or use disrespectfully.  I’d forget it.  Just study the prayer book or missal a little closer next time!  

This was me.  I have since located some information on material sin here:

Moral Theology, by John A. Mchugh and Charles J. Callan—A Project Gutenberg eBook

Thought I'd share this as well from the Baltimore Catechism #3:


Q. 285. What are sins committed without reflection or consent called?
A. Sins committed without reflection or consent are called material sins; that is, they would be formal or real sins if we knew their sinfulness at the time we committed them. Thus to eat flesh meat on a day of abstinence without knowing it to be a day of abstinence or without thinking of the prohibition, would be a material sin.

Q. 286. Do past material sins become real sins as soon as we discover their sinfulness?
A. Past material sins do not become real sins as soon as we discover their sinfulness, unless we again repeat them with full knowledge and consent.

Thought I'd share this as well from the Baltimore Catechism #3:


Q. 285. What are sins committed without reflection or consent called?
A. Sins committed without reflection or consent are called material sins; that is, they would be formal or real sins if we knew their sinfulness at the time we committed them. Thus to eat flesh meat on a day of abstinence without knowing it to be a day of abstinence or without thinking of the prohibition, would be a material sin.

Q. 286. Do past material sins become real sins as soon as we discover their sinfulness?
A. Past material sins do not become real sins as soon as we discover their sinfulness, unless we again repeat them with full knowledge and consent.

So would lustful thoughts be material sins? Sometimes it's difficult as they come with the bodies hormones so I don't always realise that my mind is thinking about women, who might be a potential wife, having/raising children (which usually leads to the thought of how children are made). In the last case I am not always sure if I took pleasure or/and consented to the thought..

Welcome to the trials of being a man, my young friend.

How am I supposed to repent and confess if I don't even know my sins? I want to be in the state of grace.