Untrained singer seeking tips on improving quickly but with maximum effort

I managed to book an outdoor gig for July 19th and 20th and am seeking some advice on how to improve in my needed areas over the next 3 weeks. I have never had any lessons but have been singing for years and years. I have great pitch but have struggled to get out of falsetto until the last few months, wherein I learned to move into modal and it’s an amazing improvement. That being said, I am encountering the following issues as I rehearse for the performance:

1. During loud, sustained notes I find that my voice will crack out of nowhere, even when the note is well within my range. I believe this has to do with not being super warmed up, something I have never done because I’m rarely singing for long. I imagine it may also have to do with breath control? Which leads me to -

2. Breath control. I am not terrible at this, but one of the songs I am singing is “Criminal,” by Fiona Apple which is outrageously demanding and I am curious if there are any especially effective ways to increase capacity. By the end of my 40 minute set I am panting like I’ve been jogging and I’ve gotta get that tightened up.

I am practicing often, warming up before each run-through, and drinking lots of fluids. I am definitely not looking for any “quick fixes,” though 19 days is of course a quick turnaround. I’m just hoping some folks will have some tried and true warm-ups/exercises to help me get in better shape for this performance which I am so excited about but also so nervous I could puke and die. :)

Thank you so much!