My DH also works long hours, similar to yours.
My DC2 (DS) is 2yo and drives me crazy. DC1 is older (school age) and I can just about cope when it's term-time, but during these holidays I have been pushed to my limits.
I also have problems with them waking each other up. Also bedtime is a nightmare because one or the other kicks off.
I too think I am struggling with depression, I did see the GP earlier in the year and was taken off the contraceptive pill I was on, which helped. Until the holidays started.
How I normally cope is with a bit of childcare (we are lucky to afford it with me as a SAHM), and then I muddle through, I probably knock back a glass of wine too many and spend too much money on online shopping. I wouldn't call myself "happy". We do have some happy times though, especially when DH is around at weekends.
Can you get your free childcare entitlement for the 3yo?
I feel very strongly that I ought to get back to work to feel better, but have no idea what kind of job would allow me to fit around school hours as there is no way DH can help with the school run.
Two boy-toddlers sounds like a nightmare. I struggle with discipline with my DS. I really do think they need their Dads around.
Look after yourself - try to see the GP.