Cousins on my dad's side: I only keep in contact with 3 of them b/c the others and I have clashed with each other personality-wise and lifestyle-wise . Cousins on my mom's side: I barely keep in touch with them b/c I have zero in common with the whole lot.

If you want to maintain contact with Cousin A and B, figure out why. Do they contribute anything to your life? Do you miss them in your life? You definitely will have to establish boundaries with anyone you want to be in an interpersonal relationship with; there's no getting around that. People need to know what you expect from them, of them, and for them. Otherwise, they won't invest any time in you.