Yes, good posture can become comfortable. It might take some time to train and build strength. After you have ingrained the new movement patterns and fortified them, you should not have any discomfort with maintaining your posture.
Before clients start working with us, they often state that their posture is only good when they are consciously thinking about it. When they are conscious about it, they will squeeze their shoulder blades together, lengthen their spine and maybe engage their abs. But this is not how you retrain the body to innately hold your posture in place. Fixing your posture requires a retraining of the hindbrain through discipline effort.
Once you figure out what good posture is, the next phase is hardening the ability to create and hold that posture. You need to do this in an environment that is extracted from your busy daily life where you can breathe, relax, and concentrate on your motions. With a mirror or camera set up to give you some feedback and dedicated effort, you’ll find you’re not often squeezing your shoulders and tucking your chin out of a sense of aggravation but rather an automatic reflex having not even thought about it. This is the point where things start to become more comfortable.