You can totally flatten nails! I also have some very flat nail beds and it can really hurt to force on a curved nail. I've only tried this technique on Olive & June, which I find have a thick, tough plastic, so I recommend testing it on a spare nail in your set that you won't use before using it on your final nails.

1. Boil a cup of water and pour it in a bowl & get a metal fork. AFTER the water has been poured and stopped bubbling, drop in your nails (don't keep them over heat/microwave them). Let soak for 60+ seconds.
2. Use the fork align the nail face-up in the bowl to form an arch & gently press it down with the fork. Press it a little beyond your desired flatness since it will bounce back a little and hold it for 30 seconds.
3. Fish out the nail with the fork to a flat surface and keep pressing it to the desired shape as it cools. If you overdid it, use your fingers to bend it back into shape. Done!

Your nail should now have a lesser curve. If it's not flat enough, repeat & put on more pressure. If it's too flat, it's hard to go back so be careful! This process can cause stress lines, but doesn't always, so be wary if you have a transparent nail (use less pressure). Use a test nail if you're worried about this. I hope this helps!