It can be really, really hard. It's like the perfect storm of tough conditions - as a mom, you're already overtired and frustrated and probably less patient than usual, and listening to a baby cry enhances all that. I find that it helps to hold on to that hope that making changes will eventually result in things getting better. Unfortunately, consistency is really key, and it can take time for things to improve. Here's some info from here:
How long will it take for PU/PD success? "If you can do this, you can expect improvement over the course of days, and a dramtic change completed with in a few weeks. Most commonly people experience a period of bad, worse, better, and then REALLY BAD again. This sudden increase in difficulty is referred to as regression. If you can stick with the method, this period will last a day or two maximum.
We recommend that you commit to a full 2 weeks of constant attention to naps, bedtime, stimulation, EVERYTHING. Try to be home for all sleeps. It takes around 2 weeks for a routine to fully take place."
Here's some info on how long to do PU/PD from here:
How long do I do PU/PD?"To start a nap the general rule is you try for 40 minutes and if they never sleep you take a 5-10 min break outside the nursery and then continue trying again. You would keep doing pu/pd until feed time arrives.
To extend a short nap you would do pu/pd for 40 minutes, take a break and try again until feed time arrives
If you never managed to get a good nap and have gotten your baby up to feed, watch you baby for tired cues and put your baby down when you see the cues."
You may want to consider whether your DD needs a tweak in her routine. I'm sorry to say that I am really not very good at figuring out routine changes! Here's my two cents, for what it's worth. It sounds like her first nap is usually one cycle and the second is a good length? Ideally, both naps would be longer than one cycle. Maybe you need a longer A time before her first nap? Have you tweaked her routine at all recently? If so, how did that go? Do you think your LO is suffering from being UT when you put her down for her first nap? Are you seeing tired cues and putting her down or are you going by her A time and the clock?
You also may want to consider whether your DDs activity time is too stimulating. I've never had a problem with this with my babies so I can't speak from experience, but some babies need less stimulation, particularly as they get towards the end of their A time.
Be careful of her getting OT. As I'm sure you know, it's a terrible cycle to get into. Don't hesitate to use an early BT if you need to.
Finally, here's one more link you may find helpful.
PU/PD Survival Strategies ~ How NOT to give up