Let's talk about happiness...
Happiness is a mental health variable.
Unhappy people are unhappy until they address that variable.
What we do can augment our happiness or our unhappiness.
If our choices are not in alignment with our desire to be happy then it's not going to happen.
If you have a goal and in striving for that goal you are absolutely miserable and unhappy then something is wrong. You either need to change your goal or reevaluate why you are doing what you are doing.
Body image issues is one of the highest causes of depression in sports.
This is why you must... MUST ask yourself if you are training hard because of something you want or because you hate something about yourself.
Yes you can use something you don't like about yourself as motivation but there's a fine line between that and it being a full blown mental health issue.
The next question that needs to be addressed is... Does the process of working towards your goal add to your happiness? If the answer is yes then stay the course. If the answer is no then reevaluate and address the issue.
I do not agree with the phrase that happiness is nothing but a choice. That's fortune cookie nonsense. The people you associate with, the things you do, the things you want out of life can all add to your measure of happiness or take away from it.