It can be 'I stayed in a relationship', but is more 'I emotionally supported you'. I guess the difference between those depends on your definition of 'relationship'! It is certainly something one friend would say to another, as much as (say) a partner would. It would be quite different to to 'I can't stand you any more'. The implication is that you gave the person too much support - perhaps that they became too reliant on you. It doesn't necessarily mean that you don't want to be in a relationship, or friendship with them any more - it could be just that you don't want them to be as reliant on you.