How often should I swim/train?

My big question is...how often should I swim/train to achieve my swim goals? I never know if swimming 5 days a week vs swimming 7 days is better because some days your body is just tired and you end up having not-as-good-workout. Does that make the workout productive or is rest better? I was swimming everyday for 3 months straight practically, working myself really hard, doing twice a week weights, and then I hurt my shoulder for the 1st time in my life! I swam competitively in high school and never had an injury. It made me feel old. However, I still want to work hard and achieve my goals but I just don't know if swimming everyday is the answer. I understand that it also depends WHAT I swim daily and how intense I work as well...but I just do whatever workout the masters coach gives us that day. So it varies. I feel like I'm training without any idea of how to get to where I want. I have a very ambitious goal...but fortunately I told myself I'd just like to achieve this some time in my life..whether it's in two years or 20 years....but I would like to drop 5 seconds in my 100 freestyle. Someday. Yes, it's obviously incredibly difficult to drop that much time in a sprint event but I want to! The question is...how to get there? Is swimming everyday the key? Or maybe it has nothing to do with number of times I work out but more about what I'm actually doing at the practice. But what should I be doing? Of course technique is important. I'd just like to hear some suggestions. I feel like I'm swimming in the dark! Thanks! -Helen