My emotions are definitely related to my girlfriend. But then, they are also internally related. I am angry at her because of the way she treats her son. I feel that I am superior to her, therefore I desire someone equal. I am frustrated because I want to leave her, but I feel so sorry for her. I am depressed because of all this.
Yes, I understand. But the solution here is not found in meditation. The solution here is found in correcting the respective attitudes & contributions to the relationship. For example, if she treats her son improperly, you have a duty to address this with her in an appropriate (ethical, loving & compassionate) manner.
The Buddha did teach the ideal partner is a partner with 'equal' beliefs (goals/values), ethics, generosity/sacrifice & wisdom however, given you are now with this woman, you can act to try to improve her & influence her to be more equal to you. The Buddha called this the 'gift of equality':
And what, monks, is the power of benevolence? There are four ways of benevolence; by gifts, by friendly speech, by helpful acts and by bestowal of equity. This is the best of gifts: the gift of Dhamma. And this is the best of friendly speech: to teach again and again Dhamma to those who wish for it and who listen attentively. And this is the best of helpful acts: to arouse, instil and strengthen faith in the unbeliever; to arouse, instil and strengthen virtue in the immoral; to arouse, instil and strengthen generosity in the niggard; to arouse, instil and strengthen wisdom in the unwise. And this is the best bestowal of equity: if a stream-winner becomes equal to a stream-winner; a once-returner equal to a once-returner; a non-returner equal to a non-returner; and an arahant equal to an arahant. This, monks, is called the power of benevolence.
~~~
But then I remember that I wasn't very happy before I met her. I guess we found each other in desparation.
This probably contributes to your unhappiness ('depression') because you probably have a sense of gratitude & obligation towards her. You have a choice to either leave her or, otherwise, help improve her.
On the one hand I want external change, on the other hand I feel that she just brings to light my inner darkness and my lack of love for myself and her.
My impression is you are expressing love, since you are angry at her because of the way she treats her son.
I am well-known for my blunt & 'sexist' comments but do not allow a woman to make you feel bad because she needs you to be like a superhero or saviour for her. Many woman urgently need to self-improve their own lives & habitual behaviours rather than expect a hero to save them.
What I am saying is it is a mutual effort between a man & a woman to fix problems (rather than blaming each other).
She says she loves me and she is very tolerant.
If she loves & is tolerant, why do you say she treats her son badly & you wish she was equal to you? Sometimes words such as 'love' are easily & not accurately spoken & sometimes 'tolerance' is a form of apathy & neglect.
In Buddhism, skilful & healthy behaviours between people are clearly-defined. Both her & your behaviours either meet the criteria or do not. If those behaviours (including towards her son) do not meet the criteria then this must inevitably cause problems & thus also provide areas for improvement & development.
In summary, it sounds like you are taking on the emotional burden, particularly blaming yourself if you leave her. However, she is also a contributor to her own fate. In Buddhism, it is taught beings are the heirs to their own actions. Therefore, if you want to fix the problem, it sounds like she must do more work rather than emotionally blackmail you that she 'loves & is tolerant'.
With metta
