Complaining about my own responses

Warm and kind words come out in real life, but when I try to write them down, they look strange, so I can only give brief responses... I create a self-imposed barrier of 'What if I say something wrong and hurt them?', which makes my responses sound odd... For things like exchange notebooks, I mostly use pre-written, fixed sentences and just copy-paste. But for thank-yous or dedicated notes that aren't like that, I end up giving such boring replies that I feel like a cold-hearted person... I'm sorry about that...