You seem to be very spiritually aware for a thirteen-year old. Good for you.
Given the content of your question, I presume you know what a “mortal” sin is - it is a sin that “kills” our spiritual soul and completely separates us from the Mercy of God. If we commit such a sin, and die without recourse to Confession, our fate is likely damnation.
There are also lesser sins, called “venial” sins, which are far less nasty. A trillion venial sins will not separate us from God (whereas just one mortal sin will do so). Habitually committing venial sins desensitizes us to the evil nature of sin, and makes mortal sin more likely, but venial sins themselves cannot result in our damnation, no matter how many we might commit.
First of all, let me assure you, that (according to Catholic dogma), there is no such thing as anything you can do that is automatically a mortal sin. If you come here and ask is a mortal sin, the answer will always be the same (regardless of what might be). The answer is always, “maybe.”
Nobody here can ever tell you if you have committed a mortal sin. In fact, you cannot ever be completely certain of this yourself (even if you become an advanced theologian with lots of knowledge and understanding about Church doctrine). Not even the Pope can know with certainty if he has committed a mortal sin.
That is because the nature of the act (the sin itself) only plays a minor part in deciding if the sin is mortal in nature. It is possible for three people to do the exact same act, and it be mortally sinful for one, and venally sinful for another, and not sinful at all for the third.
For any sin to be accountable (either venial or mortal), the person must possess both knowledge and intent. Knowledge means we understand the sinful nature of the act, and consent means we agree to it. For a sin to be mortal, both knowledge and consent must be complete and perfect.
From the content of your question, you seem to have knowledge, but not consent. But beware - we can deceive even ourselves! If you know that a video game (“Bank Heist 4”) has a transition scene of a naked girl, and you play the game intending to “block out” the naked girl, but you really take a quick look first (and this was your intent all along), you might have crossed a line.
Fortunately, as a Catholic, you have recourse to Confession, where it doesn’t matter if you were fooling yourself. That’s the really cool thing about Confession - it is guaranteed to work every time.
(FWIW, every Pope typically goes to Confession daily. The Pope has an appointed confessor who, by longstanding tradition, is a Dominican priest, although he can go to any priest if he pleases, just as we can. But, can you imagine some miscellaneous Fr. O’Reilly opening the screen and seeing the Pope sitting across from him??? He starts out, “Bless me, Father, for I have sinned. It has been about 20 hours since my last Confession…” I think that guy would wet his cassock!)