Hi all you helpful people.

Ive never really done this before so not sure where to start. I'm feeling really low at the moment, I've been like this on and off for years but always felt like i managed to pull myself out of it but realising now i have never really and its all been a front.
I feel like I should be happier than i am and wondering why i feel like this as in many ways i am lucky, i have a beautiful daughter (4) who is my world, 2 step children who visit often, a loving boyfriend and am holding down a full time job which I love. But I feel a failure, that i dont belong anywhere, i have no sense of self anymore, i just want to cry every day over nothing. I am overweight and managed to lose 2 stone last year but lost my willpower and have now put back on a stone so dont even like what i see in the mirror.
I find im not putting my all into my work anymore which may jepodise my contract so i worry about it but dont seem t be able to do anything about it. I just cant motivate myself to do anything.
I find I am less patient with my daughter (and step children), i seem to be constantly annoyed with boyfriend where it starting to cause issues but i just cant stop myself. Over the years i have lost a lot of friends so do not have my social group anymore to talk to or even just have a laugh with.
I suppose i'm asking how to get myself out of this despair i am in? Everything good that happens i just cant enjoy it as i am constantly feeling like something is bound to go wrong and i wait for it.
Im sorry for waffling on and not making much sense but this is what i seem to be like at the moment. Hoping for some advice.