My dear OP, I only started deepthroating 2.5 months ago, so all this frustration and experience is also new to me, so I can help.
At first, I too thought I'll never become good at it, I'd gag viciously with 4 inches. I thought my throat is small (I have mild dysphagia at times so I was certain it was my anatomy the obstacle). There were times I wanted to give up training, gagging, unable to stuff anything bigger than 4 inches. But I didn't. The result? I can now deepthroat 8.1 inches, I can keep it down for 60 seconds, I can breathe while keeping it down, I can massage the shaft with my tongue while keeping it down and only recently started practicing swallowing while keeping it down.
I was stuck for a month at 7.2 inches, never gave up until I found out what I need to do (relax my throat, open my throat, use my tongue, swallow, etc).
So, keep practicing and if you have a certain problem (ex. I am stuck, like there's a wall in my throat and can't go down further), tell us, we'll help.
PS. I wouldn't recommend practicing with cucumbers as nikolaos84 suggests, they are too rigid; a dick, even when erected, it's not that hard (especially a big one). >
I practice with long, peeled bananas WEARING them A CONDOM first, keeping the condom's base AND bannana's to make sure it won't slip in my throat and kill me. I try finding the longest and thickest ones. Their length are usually >8 inches but their thickness is usually not as big as a cock would. It's OK, though, should your throat becomes accustomed with something thinner, it will be easy to upgrade to something thicker. A dildo is the perfect toy for practicing but I understand if you can't have one, that's my problem as well.