Impure thoughts? Mortal sin?

I'm sorry, I know there have been so many threads asking about mortal sin but I still can't get my head wrapped around it.

How exactly would you explain 'what a mortal sin is' to a 13 year old?
My younger sister asked me what mortal sin is and I didn't know what to tell her.

Also, I recently went to confession about 2 week or so ago, and then found myself thinking impure thoughts, which I know is wrong. Since i know it is wrong and thought about it would you consider it to be a mortal sin?

Please don't tell me, 'talk to your confessor about it'...I'm a very shy type of person and find it very hard to talk to people so I don't really have a priest I feel I can talk to.

I was hesitant to receive communion but I did just this morning...

I know the three conditions for mortal sin are:

Mortal sin is a sin of grave matter
Mortal sin is committed with full knowledge of the sinner
Mortal sin is committed with deliberate consent of the sinner

But how grave is grave?

I know murder, rape, theft, and adultery are grave. But then here's the passage, "I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart." So just by thought alone, a sin is already grave?

Please enlighten me.
God bless! :)