I made a similar 'decision' as you about fifteen years ago -- my own father constantly chose his other child (now children) over my mother and I. For five years, I became isolated and turned to drugs and anti-social behaviour... then I just repressed the feelings. In the last three months, I've come to see how that situation came to dominate life and especially relationships with family, partners and friends.
Slowly working through the perspective developed to cope with the 'loss' of a parent, it's come to awareness that 1) left unresolved, these kinds of issues continually manifest is different ways and 2) it's possible to resolve personal feelings internally, without other people changing.
To the best of my understanding, left unresolved, the natural tendency of our behaviours is to replicate the environments in which we were raised. For me, that's resulted in a life up until now lined with shame, guilt and anger. Anger that my father did not adopt the role he 'should' have. However, I've come to see that the most upsetting and saddest thoughts and feelings that come up are not related to the real individual but rather a conception in mind of 'what should be' versus 'what is'.
We all have mental models of who are parents should be -- models we've learnt from parents, family, television, friends, society, etc. When reality doesn't match those models, we start to believe "there is something wrong here". Often, when that belief arrives, we tend to self identify. "There is something wrong with me because parents should love their children" or "There is something wrong with him because he should not be choosing sides".
The reality is that life is messy and your father is making the best decisions that he can for himself. It behooves you to do the same and that starts with accepting the situation exactly as it is. Really accepting it -- and that may require a kind of empathy which means putting your own feelings aside for a short while. If you can speak with him about his view of the situation and the stressors present in his own life, perhaps there will be learnings. As you listen, if you can drop the 'shoulds' and realize 'this man is my father and he is doing the best he is capable of'. If there are any following thoughts -- 'he could do better' or 'what a ******' or anything similar, that is your own mental model applying a template to the situation. Try to stay grounded in what exists before you and set your own emotions aside during the conversation and listen empathetically.
I also suggest counseling specifically around this issue. In my own situation, the 'shoulds' have wrecked multiple partner and friend relationships due to the worldview developed from the beliefs formed in from my relationship with my father. It can be a long process but there is also a wonderful side. As the 'shoulds' fall away, my memories change and instead of seeing a man who never quite gave enough, I am coming to relish the great times and appreciate them for exactly what they were. In my case, this is all happening without contacting my father at all. Because it's not about him. And the inverse of that is all the decisions that he made weren't about me.
I truly hope you find peace in this matter and free yourself from the sadness. We cannot change the past or the decisions of other people however we can prevent ourselves from repeating patterns and choose new directions.