It seems like how you feel definitely affects your speech. Like you, I had difficulty speaking with my mum when I was younger (and my dad). Fluent speech while arguing indicates that when you are impassioned the words flow. So when do you feel passion? When you are angry? When you love something? When excited? When fighting for justice? When talking to someone who genuinely listens? When expressing your authentic self? Any other times? (I'm just throwing ideas out there based on my experiences.) Figure out what gets you going, what is conducive to channelling your passion, and do those things. Figure out what inhibits your passion and creates inner conflict. See if you can resolve those conflicts, and when you do, notice any changes in how you are able to express yourself. Chances are, the more inner conflict you resolve, the more in tune you will be with your core values, and the easier it will be to express your authentic self.