And you are immediately £12,570 per year better off after tax due to the tax threshold, before you start considering the OP will be liable to the higher rate of tax.
The OP has also only become eligible for child benefit in April. That's another £1,331 per year for one child, plus an extra £881 for each additional child that you'd have got but the OP wouldn't.
So that was over £14000 per year that you got as a couple, that the OP didnt. (Not counting deductions for the higher tax rate and based on you both having just one child). Plus you have less reliance on childcare. Looking at the average data apparently six weeks school holiday is about £1000 - let's go with that being the difference between what you could cover with leave from your second adult if you had to. That's not taking into account after school clubs.
So your very basic fag paper calculation would be that as a couple you've been be at least £15,000 per year better off before you even press start.
A couple has to pay all the council tax, a single person can get a discount but this isn't 50%, it's 25%.
Yes you have one extra person to feed and water and to pension, but there is effectively a single person penalty.
Let's do the calculation on that. It's apparently on average £182 per month per person. So for a year that's an extra £2100 on food.
As I say it has improved since April but that's still in theory £13k more disposable income.
Now this assumes that the OP doesn't get child maintenance.
I've just stuck in a calculation for how much maintenance the OP would get for one child if the other parent didn't do any care, had no other children and had an income of £35000. That comes in at £4200 per year. So the OP would STILL be around £8,000 per year down on a couple. That's a reasonable income which is well above average. If he had other children or had the child on weekends it might be lower than this.
Then you start to look at long term costs. If you take out a loan or a mortgage, affordability tests will come into it. Because your numbers, are immediately better, you are going to be better placed to buy a house. Your chances of a windfall from inheritance are double. If you can afford a mortgage but someone like the OP can't, then it further gets compounded because if you are privately renting your housing costs will be at a premium and you don't acquire long term assets.
So yeah, reasonable speaking, conservatively the OP is likely AT least £6k or £7k down on a couple both on £35k a year, when you crunch the numbers before you look at housing or any sort of actual lifestyle choices even though in theory the household income before tax is the same. The difference is likely to be far more than that.
That's effectively a disposable income gap.
As I say, this is fag packet stuff, but I've trying to be reasonably conservative here rather than inflate numbers.