MuayPard

It's very possible you're just way too tight in the upper body when striking. When I first started I would tense up at the shoulders and hands and it would cause my shoulders to tire out much more quickly. At first was very difficult not to do this, but now I've been training longer my shoulders are much more relaxed and I never really have issues keeping my hands up.. Also in my experience the more I train, the less this is an issue. If you're taking 2-3 days just to recover shoulder soreness before being able to train again then something has got to be wrong there, either you're too tight, your form is making your shoulders work excessively or perhaps you're just not eating enough protein to recover properly?