It's such a tough time with everything changing and so difficult to get off the roller coaster to evaluate what is best, what works and what needs to change.
You plough through work barely putting your head up for air because you are compressing your hours, you race from work to childcare to pick up your son and then are in full on Mummy mode being the best Mummy you can be and then when he is in bed you fly through all the house tasks that have to get done which you refuse to do in baby time or work time. You need to breathe.
It sounds like your Mum is hugely helpful. Can you ask her to help on a regular basis?
Compressed hours is not always best as you work pretty long days which are exhausting, to fit this in. Consider what 90% or 80% might look like or whether better overall to go back to normal working hours with no one tutting about you not being in.
You said money is tight and I completely understand what stress that brings. Can you sit down with the numbers and see what a few adjustments might do to your situation. A nanny for us, changed my life, I could breathe and I gained nearly an hour every day by avoiding drop off and pick up plus as much as my son enjoyed nursery (he didn't actually!), he wasn't nearly so I'll and tired when he had a nanny plus she looked after him when he was sick and best of all, my stress levels didn't spike everytime the phone rang.
Work... have your responsibilities increased? Who did these tasks before?
Has the organisation shrunk elsewhere or has the business grown? Would it be reasonable to say that the business needs extra resource to cover this role? Who did your work while you were on maternity? Were they paid more? Worth looking into... If your job has changed significantly, you should be compensated for that and the additional money might afford you a nanny which might reduce your stress. Either that or the business recognises that the role cannot be done well in 37.5 hours and needs to make adjustments.
Is it worth carving some time out to work with your boss to realistically assess what the role requires and see if some tasks would fit better elsewhere or if some can be streamlined. (Sometimes taking 2 days out to do this can save half a day or more each week.) Little things like timetabling activities when you are unavailable to answer emails or phone calls. Or splitting your day, go home pick your son up, do bath and bed and then do another hour or so at home (not the best idea as this can just leave you doing more but lots of people do this.)
Talk to your Mum... if she can commit to helping you regularly for maybe even a year, it might take the stress off. You could offer a 9 day fortnight at work and your Mum could come once every two weeks maybe. In a years time, you son will get some free hours childcare and he will be more robust.
Talk to work about a phased return to work... you have already been back 5 months but it is worth having the conversation about dropping to 4 days with a view to picking up again in 6 months or a years time.
Negotiate a pay rise!! More money will make it seem worth while.
Review your finances. We always underestimate how much a baby will affect our expenditure, childcare is like a second mortgage. If you can see career progression, consider a mortgage holiday while childcare costs are at their highest, in less than 3 years he will go to school, it becomes more complicated but it will be cheaper.
Constructive dismissal is a tricky road to go down and takes a lot of energy. Are there female mentors in the wider organisation that might be able to help? Are your skills worth having? Do you see a future in the business?
You may make a change and find a role that suits you more than your current one but it is infinitely better to do this from a positive springboard than a negative one.
(I moved jobs between babies one and two from a department who didn't want a part timer on the team to one who recognised my skills and accepted my constraints. I often wonder how much the old department paid the new one to take me on but everyone was happy and my previous boss supported my move.)
You will find a way through this!!!