My 4th year of competitive swimming, a few questions

I just finished my 3rd year of competitive swimming and my Junior year of highschool. I swim for a 4A school during the highschool swim season and a not very competitive club team year round. The team I swim for in highschool is relatively competitive as far as 4A gets, and I am one of the faster swimmers on the team. I had a couple questions going into my senior year. 1. Over this past season, I dropped 4 seconds in my 100 free going from a 59 to a 55. My coach wants me at a 51 by next year. Being a small guy, (5'5") I do not have height as an advantage to drop time. Are there training techniques that anybody knows to help drop time? As in, what aspect of swimming fast should I focus on? Power, stamina, technique, stroke rate, etc. I swim 2 hrs a day 4 days a week in summer and I will be running and going to the gym as well. 2. Are there any suggestions on staying focused and motivated during practice? I have a tendency to lose focus and swim mindlessly during longer practices. 3. In the 100 free, what is more important to focus on, making that first 50 faster, or shrinking the time difference between the first and second 50? My splits are currently 26 and 29. My 50 free PR is a 24.9 4. What is the best way to tackle dropping time in the 50 free as a fairly short guy? Thanks so much for your help!