I can understand how you are feeling,on the whole i love being a lone parent but the responsibility is huge and can be overwhelming at times.
I have 5 children ranging from 7-20,my 2 oldest have left home and even though i brought them up,was there for them through thick & thin etc...they are like strangers to me now,so heartbreaking as we used to be very close 
My younger 3 are with me,i love them/do everything i can for them to provide for them and give them a safe,comfortable and fun childhood yet their dad,who they see rarely is a hero in their eyes and i know,if given the choice,they would far rather live with him. He doesn't want them though.
Hard in so many ways.
Gosh,sorry,that's not very cheerey and supportive but i think it helps to know you are not alone.....
On the positive side,i have started to make time for myself now,you can get tied up in doing everything and your life revolves around your children that you forget that you too are a person with a life to live,just building a few small things into my own life (i have a part time job now that gets me out & about),joining clubs or arranging to meet a friend for a coffee really does help to make you feel more balanced.
Hope things improve for you.