Northern vs. Southern mutations—should I pick one or just learn both?

I’m a heritage learner trying to connect with my family in Carmarthenshire, but my primary self-study material uses North Wales grammar rules. When I try to speak with my aunt, I feel like I'm using the wrong mutation patterns and it’s stopping me from sounding natural. Is it better to stick to one regional variant to build confidence, or will that confuse native speakers later? I am asking specifically about learning Welsh, not a generic study routine.