Cleaning out the attic and I found something

So my husband and I have been together for 11yrs, married for 5yrs. Things have always been great between us and I've never questioned anything in our relationship at all. Well we recently had our first child and I'm on maternity leave. I've been going a little stir crazy with not working so today I decided to clean out the attic.

It's not very full, but there's a bunch of stuff from when we were younger up there and I figured we could donate a bunch of it. I've just been going through boxes and at the bottom of a box filled with our old winter coats I found a wooden box. It wasn't locked and when I opened it I found letters, pictures, old movie tickets, etc between my husband and his best friend (male). A lot of it is sexual and the rest is obviously romantic. It dates back to 3 years before we even met.

I packed a suitcase for myself and our son and I just got in the car and started driving. I left the box on the kitchen table along with a note saying I was going to my parents place and he could reach me through my parents. I just couldn't stay in that house that we built together. I feel like my entire marriage and the last 11 years have been a sham and a lie. I'm just a cover up for whatever his sexuality is. He's been lying to me ever since I met him. I feel so sick and betrayed. I feel like I can't breathe properly anymore.

My mum always told me growing up that if I ever needed to, to just get myself and my babies to her and she'd always have a home for us. I can't believe that I actually had to walk into my childhood home with my child. I couldn't even look my parents in the eyes. I feel so stupid for not seeing signs. But I just never thought this was a possibility. He hung out with his best friend a lot, but I hang out with my friends too. I never thought it was an issue. Now I'm questioning everything. I'm so hurt and so incredibly angry. He knew when he asked me out, when he first kissed me, when he told me loved me, when he proposed, when he took vows to be with me and only me, when we had our son. He's always known. I want our marriage annulled. I want the last 11 years of my life back. I feel like he took my life away from me- my chances to find someone that actually loves me. Someone that wouldn't lie to me. I just don't know what I'm supposed to do or even say to him. He's called a few times, my parents talked to him so he knows his son is ok. I just don't even know what to do. I feel like I don't know the man I've been sleeping next to for the last 5yrs. I feel like everything in my life is a lie and how am I supposed to confront him when I don't even know what's real anymore?

UPDATE: the letters and pictures, etc date back three years before our relationship, but they CONTINUE throughout our relationship. The last letter is dated from last week and there's nude photos included of my husband and his best friend. This has been going on before and throughout our entire relationship. If it happened before and ended back then I wouldn't be so upset. Hes cheating on me with his best friend.