It's a bit hard to get the details from the way you express some of your things but I can perhaps suggest a couple of things for you. I am a competitive para-swimmer but freestyle is my stroke.

The idea of doing butterfly as a warm up stroke is, well, insane!! Its explosive and uses big, powerful muscle movements. You need to rethink your warm up routine I think. You mention your approach to your exercise has been to throw yourself into it and you clearly now realise this isn't the most sensible way to approach a training programme.

You don't have swimmer's shoulder from the problem you have described. Swimmers shoulder doesn't affect the anterior serratis muscles. It causes pain and decreased range of movement in the anterior lateral aspect of the shoulder, head of biceps area and chest muscles. It's normally an overuse condition caused by irritation. Remember also that muscle imbalance is normal - it would be very unusual to have symmetrical muscle mass. I'm not saying there isn't something wrong, just that its not swimmer's shoulder and it would be abnormal to be symetrical.

Its essential that you do two things - go and see a properly qualified and experienced sports physiotherapist for a full evaluation and diagnosis. Then once you have a programme of PT rehab to try and address whatever your issue actually is, get yourself a really good swim coach. A good coach will stop you doing yourself any further damage and can ensure that your swim technique is correct. Swimming is an amazingly technical sport and small issues can become major overuse injuries or trauma later on.

It's really hard to be any more specific but I hope you get it sorted out soon.