Other people have obviously pointed it out, but this is one of the many areas in Linux where the command line is so much easier than an interface that the people who write GUI tools just don’t bother. The tool you need for a command line approach is called dd (I imagine it stands for direct data because that is what it does). Using dd you can take data from one place and put it into another. This means you can put zeros all over a drive, wiping it in full, using
dd if=/dev/zero of=/dev/targetdevice
That will fill the whole drive with zeroes, but you could also do it with random noise first, using the below
dd if=/dev/random of=/dev/targetdevice
In the case of your ISO image there is someone who has included all the options including block size and so on, but the step you really need is to be sure you get the right device. Execuse the command below
ls /dev
Then insert your device, wait a few seconds, and run it again. You will have a list of all of the devices that were connected before and after plugging your drive in, so your drive will be the new one. It will probably show up as something like
/dev/sdc
/dev/sdc1
Notice that there are two. The first is the device, the second is the partition on the device. If you tried to put the content of an ISO image into an existing partition it would look like it had all worked but it would actually fail because the ISO is a full rip of a device, not a partition. Instead use the device itself, in this case sdc.
dd if=/path/to/image.iso of=/dev/sdc bs=4M; sync
The last bit will make your system write things to the disk and make it safe to eject it. Once that is all done it should work as a bootable USB.
It seems super complex but once you have done it a few times it becomes so easy you will regret the time spent getting a GUI installed.
If you still want a GUI you could try Gnome Disks, but I never enjoyed using it.
Cis guy here, but I work with a few trans people as a support worker and have heard a lot about their experience that may apply to you.
Puberty is dodgy for everyone. The levels go up and down over a few years for all of the growth hormones, not just estrogen and testosterone. The level of change this causes varies person to person and the timeline is not the same for everyone.
The same applies to behaviours. Some people become very interested in presenting as a given gender fairly early on, others start that later, and some don’t end up settling on a gender presentation at all or have multiple attempts to find something comfortable.
Voice training is a skill building exercise and it is really fun. Viewing it as a fun skill to learn will make it way less awful and may help to not generate dysphoria when it doesn’t make you sound fem in the first ten minutes. I don’t know how far along puberty you are and how much that has impacted your voice box but once you change the growth hormones the growth of your voice box will shift as well. It will take time for the physical shape to settle, so you need to learn how to use what you have to feel comfortable in your body. I would also recommend singing if you can as it gives you way more catchy ways of practicing a fem voice and is more fun than many of the more basic voice training exercises.
As for the fear of pushing yourself back into not doing things, yeah, it is hard. There will probably be days where it feels like that no matter what you do. There isn’t a perfect way to do it. That said, you are doing something way harder than typical puberty, you are doing puberty+, the extension course in growing up. It is harder and it will suck sometimes, but you can do it.