Hi,
Within a few weeks of taking letrozole I experienced increased anxiety and then a few more weeks later depression. This is the first time I had experienced depression, so did not initially recognise that it was until I had a conversation with my GP. Thus, for 6 years I have been taking prozac and letrozole. The prozac does not take away all of my anxiety and depression but it has reduced them to a more manageable level. It is definitely the letrozole causing your very low mood unfortunately, and if you read other chats, you will realize that you are not alone. perhaps ask your GP to reassess your prozac dose while you are taking letrozole. I am a clinical pharmacologist so do have a good understanding of how these drugs work.
Best wishes