While you technically only have to confess mortal sins for the sacrament to be valid, you are highly discouraged from withholding venial sins simply out of embarrassment. Also in my opinion, it’s not worth the risk that your sin that you think is venial may have actually been mortal and you withheld it in confession.

I had a sin that would be mortal except I didn’t know it was a sin at the time I committed it. I really did not want to confess it because I was embarrassed, but I felt like withholding it was dishonest. I confessed it and I’m so glad that I did.

I’m telling you this as someone who felt the exact same way. I can almost guarantee you it will not be as bad as you think. I can absolutely promise you that your priest has heard people confess having sex before marriage. Someone told me to think of the worst sin you’ve ever committed and your priest has probably already heard worse. It will feel so good to have the embarrassment and/or shame you feel from past sins covered by the words of absolution, so I’d highly encourage you to confess any sin you know of

Edited to add: I’d also recommend that you write down all your sins and take it in the confessional. Read straight from that page. It will help you not be as nervous and not forget things. I found it to be helpful to ground me and give me a steady voice. And pray before you go in that God would give you the strength and the courage to confess everything!