I am not sure 300 miles are enough to tell if the saddle is the problem. When I got my bike it was suggested that first I get it fit and then I should adjust the saddle to get as comfortable as I could. Once my bottom was used to riding then I should look for a new saddle if I decided I needed one. If you can’t wait then your best bet would be to find a bike shop that will let you try before you buy a new saddle. I understand there are places on the internet that will allow you to try before you buy as well.