"Sorry I am not in good condition, - a little strange this, out of context, but I assume that the writer is feeling unwell, or emotional about something... so I'd better stop here." Meaning the writer feels that they should stop writing (the letter?) now, due to the illness or emotion they are suffering from

Good condition is usually reserved for objects, not people, but in either case it probably means she does not feel well, is broken hearted or depressed perhaps.