Newbie Swimmer - Breathlessness and Mental Barriers

Hi Fellow Master's Swimmers! I'm a fairly new swimmer and started to regularly swim since the March of this year. I've been slowly working on improving my freestyle technique and my endurance. I have to say that I have seen slow improvements in my technique. focusing mainly on my balance -- keeping my head down and engaging my core to keep me balanced. I'm working on my breathing but I still have some breathlessness after swimming 25 yards. At the beginning of my swimming adventure, I was sucking air after completing a length. Today, I am winded, but not as much. Nonetheless, I still have some breathlessness. My endurance has improved as I was able to swim 200 yards total, but now swim to 600-700 yards. And my rest intervals after each length have shortened to seconds. So, I am happy about the improvements. However, I am having this mental/psychological block of continually swim with no rest. I know I can do it and I have the endurance, but I just have this barrier or maybe a fear of continually swim. What am I afraid of? And I'm not sure how to break through this mental barrier. Any suggestions? I am currently swimming 2-3 times a week. Each session is about a hour and about 600-700 yards. Most of my workout is drills on improving my balance mixed with drills to improve my pull/catch, rotations. And kicking drills too. I did take a few private lessons and was fortunate to find a great Master's team that was able to practice with and the coach has been great with giving me guidance on my technical improvement. Anyway, I've enjoyed my swimming adventure. I actually like working out and look forward to my swimming workouts! Cheers!