The good news is, you didn't do more damage. 10mph would probably have done damage to the core support and more.
The bad news is, that's not going to "pop out" with a heat gun. You're going to need to get the bumper off, lay it on a flat surface, and work the plastic back into shape from the back side.
Right now, you're just blowing hot air on it, and the form and metal bracing behind the bumper are dissipating the heat from the heat gun. All you're gonna do is burn the paint.
Here's your homework:
www.infinitiservicemanual.com
Go get the QX4 manual for your year, and go to the FWD section. That's like a table of contents, and it'll tell you which section has the front end disassembly info.
If you have problems, we'll help - but you've gotta dig in and read first.
p.s. Quit running into stuff.