

Impossible to say, could be the app is doing something funky, could be iOS, could be lotta things.
I will note, my preferred solution is to do none of the above, and I only do split DNS for one particular service. I much prefer just using an always on Wireguard VPN that is set to only route traffic to my internal subnets and to use my internal DNS server. Then I just use internal names. Wireguard basically runs at line rate on my setup, so half the time I don’t even turn it off at home. This also gives you the option to use DNS ad blocking (eg adguard) on the go.
Having been in a similar place around the same age, the things that helped me were: confiding in friends if you can, a therapist, finding the right medicine, and working on making things better one small, manageable step at a time.
Regarding medicine I know there are lots of stereotypes about what taking medicine to help with mental health issues is like, but I urge you not to write it off nor to give up if one medication doesn’t work for you. It’s a process, and one you need to work with a doctor with to find the right fit for you. Also, medicine won’t magically fix everything on its own, it’s just a little bit of help on the road to finding a complete solution.
Ultimately there are lots of good recommendations here. What works for you is going to be unique, and it’s something you’ll have the best luck with if you get some help.