One of the hardest things to come to terms with when you are dealing with a toxic family member is the 'whys'. Why do they do this, why don't they do that, why would they say something like this, why don't they seem to love me, why am I not good enough for them, why won't they listen to me/see me/hear me. You can drive yourself crazy wondering about it or trying to figure it out in the hope that if you can understand it, you can make things different.
As someone who put literally years of effort into understanding someone like this, with a lot of work you can gain insight into why they do it, but you can't change them. Ultimately all that the insight does is bring you back to the place you started at – the realisation that the two of you think very, very differently and are just very different people.
You'll go through a phase of real disheartenment when you realise that it's genuinely not about you at all. It's about them, and the way they have learned to function in the world to meet their own needs. Nothing you do or say can get them to see you better or treat you better or give more of a shit about something they really don't give a shit about.
Maybe your mother really only does love girls, but if you ask her directly she'll likely deny it and make out like you're the one with the problem. Or maybe she'll admit it and tell you she can't love boys and then make out like you're the one with the problem. And the funny thing is, you are the one with the problem. The problem is that you want someone who doesn't care and maybe can't care to care.
The only way to deal with that problem is to walk away from it. It's not easy to let something like that go, but ultimately it's the only way to live a peaceful life.