Why do I push people away, regret it and alternate between idealizing people and demonizing them?

First of all, I want to say thank you for making this question. It's something to appreciate that you made the effort to explain your current situation and the fact that you are challenging your own way of behaving. It's something very important and brave that not always is easy to do, so I send you my honest congratulations on that. 

It's normal to feel ups and downs in our life now and then. It's something that depends a lot on how are things going in our 5 main contests in life, which are work-studies, couple, social life (friends and colleagues), personal (the way I treat myself, the confidence that I have in myself, how much I love the person that I am) and family. If we go through a time when we have, from our own point of view, a lot of ups and downs, or mainly downs, perhaps it's something that is related to some permanent issues that we are having in one or more of those contexts. 

When we get very emotional or feel extremely hollow, these are good examples of not being in our best situation. From my point of view, this can be very related to this other thing that you speak about, self-esteem and self-confidence, as I was saying before is part of the personal context, probably the most important one, as it's difficult to be well with others while I am not fine with myself. When this happens, it's normal that sometimes I shut down or that I eventually explode because I am too defensive. It's completely normal and you need to know that it can be changed. 

I would recommend trying to improve the way you like yourself (your self-esteem). Try to improve your relationship with yourself. To do this, you can always talk to yourself and assess your behaviours kindly, try to keep an assertive way of communication (respecting your point of view while respecting others' points of view) and find things in your life that you would like to eventually change to make yourself happier, mainly regarding the 5 principal contexts of your life. 

I honestly think you will be able to achieve this and any other thing that you want in your life, you only need to believe in yourself and try your best.