We all have a masculine and a feminine side. The masculine side would be the part of ourselves that we view as brave, strong, stubborness, logical...etc. The feminine side would be the part of ourselves that we view as emotional, nurturing, tender, sensitive etc.
When we dream, all the different parts of our 'inner selves' interact, and 'come alive', as though they were different people altogether.

That you were female suggests you would like to be able to express yourself better, and more openly. And being younger is a need, or desire, for less responsibility. Of a time when things were simple and more carefree.

The training in the armed forces suggests your subconscious is telling you that you need more structure, and less chaos in your waking life. You were possibly trying to object to the strenuous training, but your subconscious 'chased' you off. The 'Trainer' represents someone in your waking life that is an authority figure, or a more authoritive side of you. That you killed him is a symbol of repressed anger towards this 'authority figure'...be it yourself, or a teacher, or boss... that your subconscious is acting out in your dream.
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Have you been told that you are not taking life seriously?
Or alternately, have you been 'stressing out' and feel the need for a break, or vacation?
Have you been in a situation recently where you would have liked to be able to express a 'feminine' emotion?
Have you been accused of 'being a wuss' or a 'girl', because of an expressed emotion?
Are you angry due to a situation at school, work, or home?


Hope this helps...best of luck!