I wrote a long post with some tips in it a page or two back, but after playing some more I'd do it a little different. It's not that difficult, but I think the problem is most don't realize that a 'failure' is a win too, or at least a 'win-in-progress.' Do you know what happens if you save-scum pretty much everything? You max her at 75 mind control on about day 35 and never see much of any of the punishment content, as you basically never fail. I've played it through at least three times now without savescumming or cheating of any kind and I still get to 75 Mind Control on about day 60 and enjoyed all the punishment content as well.
First thing, reduce your expectations. You should expect with no anger or lust to fail training about 75% of the time, but that will generate anger which will then allow you to punish her for cash, which you can use to buy aphrodisiacs which will give you essentially a 50% percentage point advantage at first, which will increase with successes until you cannot fail and she masturbates it away. Therefore don't worry about failures, enjoy the punishment renders and save up for your next aphrodisiac. Make use of the 50% shortcuts often, especially if you chose 'Slave' or 'Masochist' in the beginning as you don't mind generating anger--it allows you to punish her more for cash. Talk to Harley for advanced punishments early or buy the devices from Catwoman to generate more cash per hour of punishment and don't worry if you spend basically all day removing her anger as that will just make the next aphrodisiacs come faster. Don't let her end the day with more than one anger point, make sure she earns cash for you with every other point.
When you first see Black Widow make capturing her your number one priority, forget about Diana unless it's to generate cash from punishments to get more cash for ambushes. Once you have Black Widow, farm her pretty little ass all day long until you've reduced her chance of escape to minimal and have generated enough cash from excess secrets to give Diana aphrodisiacs regularly. Eventually get into a schedule where you first whip/punish away Diana's overnight anger in the morning, then give her an aphrodisiac and train her until she masturbates away her lust, then go to Widow and bang her for more secrets/cash/overfuck to prevent escape.
You've got plenty of time in my experience, just don't expect it to ever get that easy.