How do I move forward? 30 (M) regret sexual upbringing

I am 30 (M) now and my cousin is 25(M) almost 26. I feel incredibly guilty over childhood sexual experiences that continued into our teenage and young adult years. I grew up in a verbally abusive single-parent home with a father who was an alcoholic and a grandparent who was not very loving. I also encountered episodes of physical abuse and mental abuse during this time. My sister and brother were both expelled from our home and I was left alone at the age of 10/11 to figure out how to "survive" in this environment. My father's drinking began ramping up around this time as well. Around the age of 12 or 13, I was introduced to oral sex by my older brother (he was about 18 months older than me) while at our other grandma's house. I didn't think much of saying no, after all, he was my older brother and I was young and this interested me. It was also pleasurable!

A few months later my cousin who lived up the road and is just over 4 years younger than me and I were playing in our back yard and somehow things turned into "I'll show you mine if you show me yours" and we ended up showing each other our penises, and trying oral sex. I think we did this a couple days later as well. I didn't think much of this at all, since I had done it with my brother and it all felt normal and I was curious.
He also told me that he had done it with an older cousin, so that reinforced my thinking that this was somewhat normal. This behavior eventually spiraled into a regular mutual thing for the next 6-7 years. A lot of the times he would come to my house and show up in my room, and I felt expected to do this. After all their was no supervision in the home, so it was pretty easy to "get away" with this stuff. So we would just give each other head.

The older we got the less frequent this occured. It was always mutual and just something we did. I eventually told my girlfriend in college about this and she told me to get into therapy. Which I did for several years. Through therapy, I was able to understand my actions and understand that this entire thing was a painful mixture of excitement, acting out, and curiosity. I do not condone what I did. I am responsible for my actions and I am disgusted at a good majority of it. I feel like I should've known better at certain points. I feel like I will never live this down. I even got to the point where I spent hours researching on the internet, convinced I was a sex offender, and looked into what my life would be like if that were true. My anxiety was so bad, I thought I was going to be exposed. I brought forth my concerns to my cousin about 5 years ago when I was 25 and he was 21. He didn't feel like I did anything wrong, and that we were just kids and this was a mutual thing. He seems to genuinely be cool about it. I explained to him what I had learned in therapy and I apologized. He accepted my apology but told me I shouldn't feel bad because we both enjoyed it and I did nothing wrong.

Fast forward to today, 5 years later My cousin and I have a relationship. We talk occasionally as adults who live on opposite sides of the country. I have a lot of childhood issues I am working on sorting out as a result of my upbringing. My cousin grew up in a 2 parent home, and while his dad drank, he wasn't a raging alcoholic like mine. I have no relationship with my dad and I don't know my mom. I keep feeling like I can't live this down or move on from it, I keep obsessing over what we did and how If I had got caught then, I would've gotten into trouble. Or that some time in the future he's going to change his mind and turn me in. Really paranoid thoughts.

I'm getting married soon and have had a good life. I somehow made it out of a horrid situation, but not without scars or baggage. I am just trying to figure out how to process it. Should I keep ruminating on this or is it time to put it in the past and move forward? How do I do that? Am I beating myself up over nothing?

You should definitely put this in the past.
I don't think you did anything to feel guilty about. Because you were related I could see this not being your proudest moment, but you were also young and growing up in a bad situation. This is undeniably something that shouldn't hold you back from living to your full potential. For a person with OCD tendencies, I can totally see this nagging at your mind, and causing you to question all sorts of things about yourself that aren't true - causing you to ruminate a lot about it. I think it was a good idea to go to a therapist about it (and that everyone in the world should see one). I'm glad you have a good life now because this in NO WAY should stop you from that. I just hope now you're able to let it go completely. Just beware that if you do have OCD tendencies, something else will replace it as the focus of your ruminating. So be prepared for that.
All the best!