I wasn’t able to identify my core beliefs until I attended my first quality psychotherapy and it took a long time to do so in therapy and afterwards. Maybe you can attend therapy, choose a therapist who feels strongly that identifying core beliefs is essential for healing.
If you indeed believed as a child that you were responsible for your mother’s sadness, for her emotional breakdowns, then this belief is being projected into your ex girlfriend and will continue to be projected into the next girlfriend. If she is sad, you may automatically feel responsible. Before you think that you are responsible, you already feel responsible, like it is your fault.
For our mental health it is very important to correct core beliefs that need to be corrected. Otherwise we live dysfunctional lives. For us to feel okay and function well in relationships of any kind, we need to have true understanding of what it is that we are responsible for and what we are not responsible for.