“John’s so sensitive!”
“Ouch! My teeth are really sensitive!”
If you (or your teeth) are sensitive, it means they have a lot of feeling. They’re really full of feeling. That is, they are feelingful.
“You’re more feelingful than your brother”
“He’s the most feelingful person I know!”
NOT: feelingfuller, feelingfullest. We don’t say “resentfuller” or “beautifullest”.
The negative can be formed with un-: unfeelingful. That is, “insensitive”.
That’s it! A new English word for you formed totally regularly from the tools already available to us.
© 2020 Bryan A. J. Parry
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Wrixlings 
A very nice new word, which also shows that English does indeed have the needed tools – it just has to brook them more! Theech has the orbeteed (“cognate”, or “homologous” to brook a word from the beholding of unfolding / theory of evolution) word “gef́ühlvoll”, but this has a slightly other meaning. The narrowkirily same-meaning Theech word as “sensitive” is “ɂempf́indlich”.
However, I have two intings/issues:
Firstly, I would indeed say “feelingfullest” rather than “most feelingful”, just as I’d say “wonderfullest” rather than “most wonderful”. The Theech-speaker also doesn’t say “das ɂam ḿeisten ẃunderv̀olle”, but “das ẃunderv̀ollste” instead. It’s narrowkirily here, in the speechcraft/grammar, that we also have to forenglish the speech more, I think. Indeed, I’m starting to say “beautifullest” (better still: “fairest”) and the like even in plain Engnlish sometimes.
Secondly, I think that there’s nothing wrong with a bit of Latin. If thou askest me, I’d brook “sensation” for bodily sensations, and “feeling” for mindy and soulish feelings; so I’s say “Thou’rt feelingfuller than thy brother”, but “My teeth are sensitive”.
What dost thou think?
*plain English
Interesting thoughts. I would say that in Standard English, words ending in “-ful” take the comparative in “more”, not “-er”, but feel free to play with the language as you want. Certainly, I enjoy the -er comparative forms, and it’s one thing I like about Swedish. As far as being okay with Latin words, this blog explicitly states that I don’t believe in taking out all non-homeborn words. I have several posts where I talk about this head-on or in a side-note way. Thank you for the thoughtful comments!
ancient man would never say stupid general-meaning word.
What?