You won’t really shock him.
Trust me on this. Unless your priest was ordained the day before yesterday, he has heard it all. You will not shock him. You may think your situation and your sins are unique. Probably not.

I have broken all ten commandments. I will shock him.
Come on. Your sins are not that original.

No, really. You have no idea.
That is your pride talking. You need to confess that, now, too.

I haven’t been inside a church, much less in a confessional, in years. I don’t know if I’ll shock him. But I am scared.
There is no time like the present. Just go in and tell him that you haven’t been in years. Tell him your mortal sins: missing Mass for 20+ years; major sexual sins; abortions; stuff like that. He will probably ask you a few questions, to make sure you aren’t holding back anything major. He may make suggestions for avoiding sin in the future. He will probably mention the Prodigal Son or the Lost Sheep, and Heaven Rejoicing.

You make it sound easy. I don’t believe you.
I was away from the Church for 12 years. I admit I was scared about my first return confession. I only went in desperation. But it was a huge relief. And I do feel sure my family is waaaaay better off with me going to confession regularly.

I’m still scared.
You will do fine.
You won’t shock him. You will delight him. And me. And the Lord.