It sounds like you are dealing with much greater problems than I ever had to deal with so I'm not sure my suggestions will have any validity to you, but here are my thoughts on the issue nevertheless...
Make sure you are getting enough exercise. A 20 or 30 minute walk in the evenings can help even out the mood swings.
If you are a vegetarian, make sure you take B-12 and B-Complex vitamins. Even if you are not, this might help. Everytime I forget to take them for about 2 days I become inexplicably depressed.
Make a list of the things that you can actively do to improve your life in the next day, week, month, year etc. Make sure those things are small enough tasks that no one task becomes so overwhelming that you give up in dispear. (For example my job has become boring and non-creative. This weekend I worked out a plan for developing my skills in a creative area and using them to either improve my current job or find a new and better one.)
And about the 15 year old; you can find her attractive all you like, but you can't do anything about it. Painful and frustrating as that may be, just recognize that whatever you do in regard to her is a choice you make and you are responsible for that choice. You have already indicated that you know what the right choice is, so you have to live with that.
Good luck
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Never let the weeds get higher than the garden,
Always keep a sapphire in your mind.
(Tom Waits "Get Behind the Mule")