Pickles, I have just read this whole thread, and I wanted to say how strong you sound. It is incredibly early for you to having to fight these battles and deal with all this crap, and you are doing really well.
I understand some of what you're going through. My DS's dad left when he was 6 months old, and was very unreliable with contact. I wanted our son to have contact with his dad, so I put an amazing amount of effort into it... I did not drive but I travelled by train and bus to take DS to see him; and I even took Ex to court to try to get an official contact order (I failed). When he moved 150 miles away, I still continued to take DS to see him a couple of times a year. His dad did drive, but rarely made the journey back here. With hindsight (this was all 13-17 years ago) I don't really know why I did it, except that he used to say that I was 'stopping him from seeing his son' - and like you I felt somehow guilty. 
Also like you, DS's dad decided the maintenance he was paying was 'too much' and cut it down to £30/week, then after 2 months, stopped it altogether. In 17.5 years I have had a total of about £250 from him.
:(
Don't be fooled into thinking your DD's dad will suddenly become caring and supportive. He won't.
When my DS was 13/14, the CSA finally caught up with his dad and made an attachment of earnings. He quit his job and left the country. He wrote to me telling me I had 'forced' him to go abroad. He wrote to DS telling him the same, and also saying I had 'tricked' him into having a child in the first place.
I don't even think it occurred to him to consider how this would affect DS. That kind of selfish fecker can't see past the end of his own nose.
I had times when I was on my own with DS when I was very stressed and unhappy and depressed. Stress and grief and lack of sleep do mess with your head.
(I'll send you a PM saying a few other things about this that I don't want to say 'publicly'. :) )
You will be able to deal with this. Take any bit of help you are offered, and if you aren't offered enough, ask.
You can do this. If you can be this strong when your DD is just a few weeks old, just imagine how strong you will be by the time she is one... two... five... ten... :)