Excercise no longer keeping depression/anxiety at bay..

Hi - basically I started working out five or so times a week from the beginning of last year but not to keep physically fit. I did it at the recommendation of my doctor as I had been having anxiety attacks and bouts of feeling hopeless. It really worked well, and as a side effect, I got into great shape. I do high intensity workouts at least five days a week. The effects used to last until the next day and then I'd continue with it to keep the effects going. Now something odd is happening - I feel great when working out and then an hour after I come crashing down into these horrible lows. I feel like absolute crap afterwards and hopeless about the future. e.g. today I got back from the gym and while showering I started to think about how we are all going to die anyway and life is pointless. In short - what used to be my 'cure' for my own issues seems to have stopped working :( Is there any reason for this? Could I be exercising too much or be relying on my 'fix' too much? Thanks

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