I've been depressed too. From inside it, it can look as if you can never fix it.
Your mind seems to be very like mine in some ways, with similar strengths and weaknesses, so I might be able to give you special pointers.
Understand that depression talks, and does not tell the truth.
You may be taking what you think is a rational survey of your situation and it is not in fact rational, but rather it is depressive.
For example, say someone lost a lover. They say, 'I'll never find that person again, it will never be the same again, I can never find that happiness again'.
This is false. It's depressively false. It's true in the sense that you will not find the very same thing again. It is false though that happiness can no longer be found. It always can, but not in the very same place, or very same form, that it was in before.
People may tell you to 'look on the bright side' and it may be useless to you. You may need to instead learn when a thought is depressively false.
Those thoughts that tell you that 'if you had the last three years again, you would be happy but otherwise you cannot' -- these are false. They are mistakes. They are wrong. Understand that. Understand that your mind is actually making mistakes because you are depressed.
Understand your mind as a thing which makes mistakes (and forgive it for it). When you think these thoughts, note that your mind has made a mistake again. See it as if it has made a mistake in a calculation, as if you had just wrote 2 + 2 = 3. Be loving towards it. Say 'there it goes, making mistakes again, silly mind'.
This is only one aspect of the 'fix'. Fixing depression, like keeping a garden, is not generally done in one move. It is done in a long sequence of sustained moves.
Here is another 'move' for you. Try this drill on occasion:
Just feel whatever you feel. You have a fast, agile mind and you are probably constantly using your mind to defend yourself from things that feel unpleasant. Possibly always looking for a way out from what feels unpleasant, mind getting more agitated and over-worked in the process. Reverse that. Pull right into your feelings, even if unpleasant. Feel the unpleasantness of it. The sadness. Most of the world is in there with you -- sadness is common. Quit trying to get rid of it by thinking. Have a good strong feel of it, and note that that feeling is part of what it is to be human and to have experienced life. You may get a sense, 'this is sadness, this is pain, not as scary as I thought'. And when you get less scared of sadness and pain, your mind may stop exhausting itself and making itself worse by continuously trying to dart away from it.
