I can remember my DD telling me about ex and new g/f having evenings out, using local gym for saunas and swims, social nights at the pub....whilst I sorted laundry, supervised homework, listened to tantrums (which escalated because DC were unsettled) and struggled to make ends meet.
I wish I could say that is either ok or rare but I think it's fairly typical for many lone parents. Sorry.
The best advice I can give is a blunt 'get over it' :) - I have spent hours bitterly considering how unfair this situation is to me, to the DC and to society. There is no solution. It really is a situation of making sure he pays what you can get him to pay in terms of his financial responsibility and then just accept it and don't waste time on thinking about it.
My eldest doesn't see her Dad by choice. He's apparently gutted by this. (Reap as you sow.) Does this make me happy? No because I still feel a father's role is important to a child and I feel for her because she doesn't have one. It does balance out some of my bitterness at how his life seemed so easy when mine was hard work.
Overall, remember not all men are like this.
I have a new partner. He's a fantastic father to his DC. He offers my youngest DC what she is missing in terms of a father (we all know he isn't her Dad but its nice to have a figure offering that role) He also adores me, treats me, tells me I'm gorgeous, cooks and cleans up after himself. I am still hugely wary of him because I cannot believe a man like this exists! What's the catch?
Meanwhile my house is mine. I do what I like. My relationship with DC is rock solid and I feel blessed. Those first few years I remember as very hard and unfair. They were, can't change that, but it won't always feel like this.