I had this happen a long time ago. I am sorry to hear you are dealing with this.
It sounds like your iPhone SE is experiencing a significant issue, possibly related to the battery or the system software. When an iPhone restarts frequently and struggles with battery life, it could be due to a degraded battery, especially if the phone is an older model. However, the inability to update and the storage being full for a long time could also indicate software corruption or other system issues.
Here are some steps you can try to resolve the issue:
1. Backup Your Data. If possible, back up your iPhone using iCloud. Be sure that you buy enough iCloud storage or a computer. This ensures you don't lose any critical information if further troubleshooting is required.
2. Force Restart Sometimes, forcing a restart can help with software glitches. To force restart an iPhone SE, press and hold both the home and the top (or side) buttons until you see the Apple logo.
3. Update via iTunes/Finder. If you have a computer, try updating your iPhone through iTunes (on Windows or macOS Mojave and earlier) or Finder (on macOS Catalina and later). This can sometimes bypass issues that prevent updates from occurring on the device itself.
4. Check Battery Health If you can access the settings on your iPhone, go to Battery > Battery Health to check if the battery may need replacing.
5. Free Up More Space
If your storage has been full for a while, it might be beneficial to free up more space than you think is needed. The iPhone requires free space to operate correctly, especially for updates.
6. DFU Restore
As a last resort, you can put your iPhone into DFU (Device Firmware Update) mode and restore it. This wipes the device completely and installs the latest iOS version. Be aware that this will erase all data on your iPhone, so it should only be done if you have a recent backup.
7. Contact Apple Support
If none of these steps work, there may be a hardware issue with your phone. Contacting Apple Support or visiting an Apple Store for a diagnostic could be the next best step.
Please note that if the battery is the culprit, the iPhone might continue to restart even after a software update so it may need a battery replacement.
In your case, considering the update issues and the frequent shutdowns, both the system software and the battery health may be contributing to the problem. A visit to an Apple Store or an authorized service provider might be inevitable.
Keep in mind this is based on the information you've given, and without hands-on diagnostics, it's a best-guess scenario. If you're comfortable with these steps, you might be able to resolve the issue at home. Otherwise, professional support would be the way to go.
PS
The iPhone SE came out in 2016 I would check with your cell provider if there is a good chance that you can replace it with a new iPhone 15 or an older iPhone of your choice. Because of the new launch, you might get a very good deal on a new phone.
