Sorry, I'm confused now. In your opening post you said and she has a chromosome issue which will be with her the rest of her life!! but then in your 2nd post you said No she has not been diagnosed with a chromosome disorder.
In your op, you said My DD has been diagnosed with Speech Sound Disorder. but then in your 2nd post you said I had an Educational Psychologist to assess her and got a report saying "she thinks" she has Speech Sound Disorder An EP can't diagnose, and it appears she has only said "she thinks...". What does the SaLT report say?
I'm also puzzled about how an EP has seen her, if the Nursery haven't had conversations with you about her delay, or haven't had plans in place to support her in Nursery.
Is it better for her to get started or give her another year to catch up a little If she is given one of the places in the Unit, then accept there and then. The earlier intervention starts the better.
But what do I do if she does not make it into the Speech unit? Feeling so stressed as we all want to do our best for our children
It's difficult to know what to advise you, as I don't know what the options are where you live, but you should be getting support and advice from the SENCo in the Nursery (SENCo = Special Educational Needs Co-ordinator). I would ask for an appointment to speak to them and ask why it is that your dd hasn't got an individual plan in place to support her. I would also ask - both the SENCo and the EP - about applying for an EHC (Education, Health Care Plan).
If you need additional support, every Local Authority have to have SEN (Special Educational Needs) Parent Support Service SENDIASS Do a google putting in SENDIASS and the name of your local authority. The staff there will know more about your area specifically.
However, there are a lot of knowledgeable people on these boards so feel free to come and ask more questions.