I hate on myself a lot and saying positive shit about myself in those angry moments we get pisses me off greatly
The way i broke it down for my own mind is by asking myself why positive self talk makes me so angry.
What i found was that due to being isolated, ostracized, bullied, sheltered, etc as a child, my entire perception and understanding of the world has caused me to form a pattern of logic that would make me hating myself the logical thing to do. to say positive things about myself in those instances literally feels illogical, and to do the illogical makes me feel like im trying to play myself for a fool. It’s like as if someone is trying to convince you the sky is blue when your pattern of logic has convinced you for years that it’s actually green. 
I have found that a good first step in positive self-talk is to notice what you think about yourself/your partner/your peers, and then ask yourself why you think that way.  
For example:
“Im stupid, i’m terrible, i suck, im the worst.”
Well, why am I the worst?
“Because people don’t like me, because i’m stuck in a job I hate, because I didn’t do well in school, (etc)”
Why does that make you the worst tho?
“Because everybody else, my FP, my peers, are all doing better than me”
Well, why do I have to be like them in order to not be stupid/suck?
And that’s when i realized that I felt the way I did because my only metric ever for success and normalcy was other people, because I never had friends as a child that accepted me for who I was and validated my existence, both flaws and talents… and by the time I was socializing the imposter syndrome had already set in and I have mimicked other people’s personalities and actions ever since. In other words, the basis for my logic has been influenced by neglect and abuse, and is not a true reflection of my world or myself. 

I still don’t like positive self talk, but i feel better now that I realize that it’s my system of logic that is flawed, not me. And even though in the moment I hate myself, I cool down later and I talk to my boyfriend, or write it down in my diary, and then i move on and try again next time.