I don't think you are letting her down, you said you spend lots of time playing with her, and that IMHO is the most important thing about this age.
Don't feel pressurised into rushing the teaching of reading & writing at the preschool stage, just make sure you read lots of books together and show her what fun reading can be.
When the time is right, you can introduce recognition of letters and numbers really gently. For example, when you are out and about, you could point out the odd number and letter (house numbers, street signs) if she seems receptive to that. Or make her initial letter out of playdough, put her name on her bedroom door, etc. If your DD can recognise a few symbols/digits/logos, then she might be ready for that sort of thing.
To facilitate writing, use games that encourage good hand-eye coordination - teach her to catch a ball for example, and do activities that help strengthen her hand muscles and make her more dextrous, e.g. cutting out, lego, thread cords through card with holes in etc.
There is lots of scare-mongering about teaching reading using various phonics methods at an early age, but unless your child has specific learning difficulties, she will probably learn through a combination of whole word recognition, some phonic de-coding and blending, and clues from the context (i.e. pictures, general knowledge about the topic). In some other European countries, formal teaching of reading & writing does not begin until age 7, and their illiteracy rates are not as high as in the UK - so early teaching of reading & writing are not necessarily the most important things.