How come I only have insomnia during the summer?

Sunlight. Light is the strongest cue the brain receives to stay awake, it is more powerful than any other cue or "zeitgiber", summer has more light and prolonged days possibly affecting your circadian rhythm.
If did not try. It is normal for your body to start to wake up when exposed to sunlight. There are many things you can try to sleep longer in the morning. The first is, as you suggested, a sleep mask which you put over your eyes to block out the light. Make sure you find one that is comfortable so that it does not keep you up during the night. Another option is to buy room darkening .If did not help try melatonin.