I also often think that one should stick to one school of thought, to one teaching etc., but often this leads to narrow mindedness and rigidity. Besides, if a given method works for you better than another, why don't use it?

To quote:

> “Those who teach a Dhamma for the abandoning of passion, for the abandoning of aversion, for the abandoning of delusion — their Dhamma is well-taught”.
(Ājīvaka Sutta; AN 3:72)

Firstly, we can see that the Buddha taught in a manner that is conducive both to oneself and to others. There is often the impression that Dhamma is exclusively concerned for other's, but that's not quite correct:

> Whenever you want to do a bodily action, you should reflect on it: 'This bodily action I want to do — would it lead to **self-affliction**, to the affliction of others, or to both? Would it be an unskillful bodily action, with painful consequences, painful results?' If, on reflection, you know that it would lead to self-affliction, to the affliction of others, or to both; it would be an unskillful bodily action with painful consequences, painful results, then any bodily action of that sort is absolutely unfit for you to do. 


Secondly, if we take MN 20 for example we can clearly see that Buddha employed a number of strategies not just one. "If X  doesn't work use Y, and if that doesn't work either, use Z".


And lastly, there is the pali word 'upaya', which translates as 'skilful means'. When the Buddha was asked about certain things or teachings the Buddha often gave 'mundane answers' or remained silent, in order to not cause further confusion. Here too, if Mindfulness of Breathing causes you distress then put it aside for a while and do metta meditations

Modern psychotherapy too uses often a pluralistic, that is, eclectic approach, in order to help the client as effectively as possible. 

To summarise, there is no one size fits all. Different people, different history, different dispositions, different preferences & so on.