There really is no easy answer OP. I really do feel for you. I think a lot of women who work feel the same 
One thing I would point out is that being a SAHM does not mean that you have to spend loads of money on activities!
Parks are free. Crafty sessions at home are free. Swimming is still free (I think for kids?). You may meet other mums and go round to each others houses which is free.
ds2 is 2 and is only just starting swimming lessons this week. ds1 has lots of activities but he is 8 in june and didnt do any til he was nearly 6.
If you gave up work or got a different job/less hours what could you REALISTICALLY do without?
Would you need 2 cars?
Could you downsize your property?
Could you cut back on utility bills?
Are there any luxuries you could do without? ie. cable/satelite TV/meals out etc?
Can you do without at holiday or go camping instead?
(sorry if these have already been mentioned!)
I have been a SAHM for 8 years and (generally!) love it
but it has meant that we have made sacrifices...
we go on hols in this country - self catering. No more long haul for us!!! 
we dont really go out much
we dont spend much on ourselves
we dont smoke or drink
we have a car but own it (its 6 years old and I will be driving it til the wheels fall off!)
instead of moving to a larger property we have done up this house instead
our only luxury is cable TV!!!!
I know that when I worked I spent so much of my wages on petrol/public transport, new clothes/shoes for work, pack up to take to work etc etc I was amazed when I added it all up.
I wont lie...I sometimes wish I had my own money...I sometimes wish that the financial burden was not all on my dh BUT the benefits have far outweighed the cons for me. If my kids are ill, I dont have the stress of what to do. I can do the school runs, go to plays, school functions, have 1-1 time with ds2 whilst ds1 is at school.
I also volunteer as an assistant beaver leader too which I love.
I really hope you can come up with a plan that will work for you x