Welcome to eGullet, gary.
My personal philosophy when it comes to Thai (or most other cooking) is that one takes general direction from cookbooks, learns the basic techniques, ingredient tastes and combinations, experiments to find the mix that works for oneself, and then relaxes. After all, most Thai cooking/recipes/types of recipes are just the result of home cooking - and the taste would be different in every home you visit over there - or every restaurant you visit here or there for that matter. Learn the general technique and what a dish might look like .. and then use whatever ingredients you have that might get you there or close. The purpose of cooking is to make what one is going to enjoy eating - and everyone has different tastebuds so how can you be 'incorrect' when it comes to flavour if YOU like it?
You might try mixing up your curry paste in a blender rather than trying to do it all manually in a mortar. There really are no prizes for killing yourself to recreate a paste that even in smaller places in Thailand today is often done by machine too. And indeed it seems that many home cooks in Thailand are now using premade pastes as well and say there is no shame in that.
Taste as you go along ... if you don't like what you taste .. adjust it - add more chilis or lemongrass, etc. And there is another point - you may be using 'authentic' ingredients but they may not have the same pungency as they would have if just pulled from a Thai field, etc. so you just need to develop both your own taste and a feel for what the end product is you want to produce. Keep practicing.
