Your phone appears to have significant and actual physical damage, something that no app you download from the Play Store can fix. Software solutions can only do so much to alleviate hardware problems.
You stated you don't want to send the phone out to be repaired but you really don't have a lot of options for this. The display will not heal itself, it needs to be replaced.
Unfortunately smartphones are not by design manufactured for us to readily repair them. While there is a movement to change this, it's years down the road. So replacing something like a smartphone display requires an almost total tear-down. Depending on your skill level and how comfortable you are taking your phone apart it's quite a daunting project.
The iFixit web site has a display replacement guide for your model.
www.ifixit.com
As you can see, it's not a simple task. You need all the proper tools and be comfortable disassembling/reassembling a device not intended to done by consumers in general. And if you've never done it, doing so on a primary, daily usage phone is not recommended. It's better done on a secondary spare phone as practice.
Anyway, if replacing the display isn't doable (it's not for most people), and you don't want to send the phone out to be repaired, your options are live with the screen as is and hope it doesn't get worse, buy a new Revvl Plus phone as an exact replacement so your Dad doesn't know, or have your current one repaired at a trusted service center and hope your Dad doesn't find out. (Or is it too much to expect that Dad's love for his daughter is more than a damaged phone?)
Do you keep your phone in a case? Or yes you do and this damage still occurred?