I would like to try and correct a misunderstanding that you have. It reads to me that you think that because before you blamed others, it means that you did not have the core belief then that you are guilty for what you are not.
It is not so: when a person feels guilty, overly guilty, on an ongoing basis, the person takes breaks from self blaming and blames others.
People go from one extreme (blaming oneself extensively) to the other extreme (blaming others). This happens because people don’t see another option, which is to take responsibility for what you are responsible for and to give others the responsibility that they do own.
It is similar to another extreme people go through: passive people go from that extreme to being aggressive, taking a break from being passive, not realizing that there is another option: to be assertive, the middle, healthy way.
And so, I believe your core belief of being responsible for what you are not responsible existed since you were a child. I believe it is not an issue of adulthood.
And so, this core belief was formed in childhood, I strongly believe in your relationship with one (or two) of your parents. This is when and where most of our core beliefs are formed.
Somehow, you will need to go back there and settle the issue of responsibility there. In other words, it will come to this: you will find out that your parent/s are not as wonderful as you thought they were. What will be the advantage of that, you might ask:
The advantage will be that as they were not as wonderful as you would like to believe that they were/ are, you were not as faulty, wrong and guilty as you believe you are.