Oh, so glad you've taken this on board. It must have been a bit of a shock but glad you've seen that it was a lovely idyll but one that was highly unlikely to get you what you wanted.
Thinking about this a bit more you sound like me when DS was aged 1-2 - I was drowning in work (was in practice at the time) and working late nights, weekends etc. It was soul-destroying, I felt I was failing as a mum and as a professional. I wonder if it's a thing that happens fairly often at this stage - I definitely think you can 'have it all' as a parent and an accountant but working out what 'it all' looks like may require some readjustment. This all sounds a bit naff and patronising and I'm not given to introspection much but looking back (with the benefit of 10 years hindsight now) I think part of my problem was I was fighting against adjusting my expectations and also what I was physically capable of and wanted to do.
For me it was also realising that asking for help wasn't a bad thing. I just kept accepting more work and not saying I couldn't get it done. I was worried that if I did that I'd be badged as 'just a mum' who wasn't committed to my job - what I hadn't realised was that anyone would have asked for help in that situation. Accountancy can be brutal in this respect as for the first 10 years of your career (assuming you're ACA/ACCA) you're fighting to get qualified, putting in all the hours to get promoted etc and it just keeps going.
I did two things - firstly I realised when I was in the office on yet another Saturday afternoon that things had got beyond what I could manage - I used the rest of the day to pull together a list of everything that I had to manage, pointed out how many hours it was taking and said that there was too much work. I got a secondee manager within a month to help. Secondly I stopped saying yes to everything I was asked to do and started saying 'no I don't have the time available for it' and then negotiating the alternatives eg someone else to do it, different deadlines, agreeing that something else was to take a back seat etc. I think this is a 'maturity' (not quite the right word but hopefully yswim) that you get around that level in any case - I certainly saw it in other colleagues as well who didn't have kdis so it's not just a 'mum thing'.
Longer term I trod the well worn path of jumping ship to a client - because they wanted me I was able to negotiate flexible working from the beginning and that stopped the long away jobs/endless travel etc. My career has continued to progress with promotions, surviving redundancy rounds etc.
Although all finance role have peaks and troughs, my experience has definitely been that if you're good at your role a firm/company will want to keep you and provided you have sensible arrangements/plans for your work (and that's not checking email/phone constantly outside of working hours - that's taking a pay cut for the same amount of work!) then you should be able to find something that works whether it's compressed weeks, annualised hours, TOIL, extra holiday etc.
Bear in mind as well that your needs will change as your child grows up. Whilst they're at nursery flex/toil might be best and will give you down time/the ability to be around if things go wrong etc or a guaranteed no. of evenings etc. Once they start school flexibility to attend school events can be great (I used TOIL) but you might prefer shorter days so you're there for after school or longer holidays to cover the endless weeks of school hols.
Sorry - this has been an epic post but I hope it helps to see there are options and that in my experience where you are now and the casting around for something else is not unusual.