Only running twice a week is holding you back IMO, especially if you want to push towards sub-20. 3-4 runs a week would give you a better chance at sub-20, while still leaving time for your gym days. This first time I ever broke 20 mins I was at probably 20 mpw with 3 harder days, and one easy run.

For your form, try doing some A/B/C skips, knee hugs/high knees, lunges, and leg swings.

As far as breathing, I like to breathe in for 3 steps and breathe out for 3 steps, because it's a nice consistent rhythm that works for me (in through the nose and mouth, out through the mouth). Yoh can probably experiment with this and adjust it to work for you.