Terrified of miscarrying

i am 5 weeks pregnant, i had my last period on the 7th sept.
i have planned for this baby (its my last one) and i keep panicking about miscarrying. i have constant lower back pain.
i am always tired and weepy. im hoping this is not permanent through out my pregnancy because i cant rememebr feeling like this with my last 2.
i keep thinking ill be fine if i make it to 8 weeks, then 12 weeks.
i know its just nerves but ive had to be signed off my work because i think it will be too much.
does anyoe else feel like this or been like this?
kirsty