It's really difficult at times.
I've been a single parent for nearly 6 years, during which time DD has gone from a relatively inexpensive 11yo to 17yo.
My parents have helped out a fair bit...they paid for her to have a mobile when she started secondary school. I took over the contract when she was 15 and wanted something more 'with it', and by the time.she upgraded last year, she had two part time jobs so now pays for it herself.
Clothes get more expensive, luckily DD hasn't been massively into branded stuff other than shoes - she's tall and a size 9 so gets those those birthdays/Christmas. She buys most of her clothes now and also pays for her own gym membership.
She gets a bursary for her college travel expenses - a bus card which she can use 24/7 and is a absolute godsend.
Driving lessons - birthday present from my mum who paid for 10 hours worth, she saved some of her wages and Christmas money for for the next few. I've paid for her theory test.
Uni is a worry. We'll be doing open days this summer...4 day trips to London and two weekends to unis further afield. XH has declined to get involved and I've not been working for nearly 18 months due to becoming a full time carer for my elderly parents, so none too sure where the money for that will come from.
As for uni itself, she'll get full loan, and I have got a decent amount if equity in our house so am prepared to downsize to a 2 bed flat if necessary.
The other thing I did while I was working was start a SAYE account...having between £10-£50 a month deducted from wages.
It wasn't much but it meant the cash was there when she needed secondary school uniform, school trips etc.