Izzy, I was in a situation with my grandfather several years back that caused me a lot of guilt and heartache. He had lung cancer, and was living with my mother. She had been taking care of him for awhile and needed a bit of a break from it, so she asked if I would sit with him for the weekend while she went upstate. At that point he was pretty much self sufficient, so it was really no big deal.
Well, as the fates would have it, he plummeted that weekend, and went from self sufficient to not even being able to get out of bed. I was only 25 years old at the time and had NO IDEA what to do. Long story short, he died a few days later and I lived with MAJOR guilt that I should have done something more for him during the time we had together. Thoughts and memories from that weekend haunted my dreams for a few years afterwards. Guilt ridden, anxious, and horrible dreams, which like you, I couldn't control with mere logic.
Then an amazing thing happened one night.... I had a dream about him that was completely different in nature. I don't know what your beliefs are about life after death, but I'm of the belief that the people who die before us still keep an eye on us. Well, I think my grandfather knew I was completely twisted with guilt over that weekend, and came to me in the most vivid dream I've ever had....
He walked down a hallway towards me and gave me a hug. He looked down at me with a smile, and said "Okay.... you've got ONE more week to think about all that stuff, and then you're going to let it go!" And then I woke up.
I swear, it was like having him right there in the room with me. So, I did what he told me - for the next week I pondered all the things about that weekend that went wrong, and then tried to think about the handful of things I did that went right (something I hadn't tried before). And then after that week was over, I never had another one of those nightmares about him again. Weird, huh?
I think regrets are okay to have, they are one of the tools we use to learn by. To say, "okay I would like to do things a bit different the next time around" isn't a bad thing. But we can't let the regrets hang over our heads for too long because then they stop being a learning tool and start to make us afraid to try again.
I don't know the whole story with regards to your pregnancy, Iz, but it sounds to me like there wasn't anything you could have done to prevent the miscarriage. I lived through a miscarriage myself, and thought of a million reasons why I might have caused it, but when it comes right down to it, it's just horrible luck.
I only know you by your posts, but it is obvious to me that you have an amazing heart, Iz. It's my prayer that you can somehow find a way to glean some good memories from the painful ones, and let them fill your dreams instead.

Maggie