Is onanism a mortal sin?

Let me give a bit of context.

I'm young, I'm not married. I have been sinning of onanism and lustful thoughs almost daily since my teenage years. It's hard for me to control it.

because of this, I have refrained from receiving the communion for most of my life, since I consider onanism a mortal sin. And also, I have refrained from confessing most of the time, since I don't think I'm strong enought to fight this vice, and thus, I'm not trully repentant. I only confess once a year, during lent, because I think not confessing ever is even worse.

Is this normal? Do people consider onanism a venial sin and receive the holy communion if they sinned? I see lots of young catholics not receiving the communion when they get girlfriends, because of concubinism/fornication, but I have never heard of people refusing to receive the communion for masturbating.

I think it's a mortal sin, because in the end, I know it's wrong, and I keep doing it. But I feel this is a vice lots of people fight, and I think most young catholics either receive the communion and don't care, or they treat it as a venial sin for reasons I don't understand.

When I confess this every year, the priest always ask me "Are you married?" and when I say no, he gives me a small penitence. So I'm not sure if it can be considered venial when not married. I'm a bit confused about the whole situation.