Lately, I suffer from an unusual habit. I simply don't finish my sentence while typing and I don't even notice it. Afterwards when I take a look at (for instance) the email I sent, I can clearly see that I've stopped in the middle of the sentence. I wouldn't believe my colleagues if I couldn't see my email history. So I'm wondering if
When talking, this is usually referred to as "trailing off", or "trailing off in the middle of a sentence". Eg
"Susan started telling John about the weekly sales report, then trailed off, looking over his shoulder."
"I'm always trailing off halfway through a sentence, especially when I haven't had a coffee yet."
Alternately, the trailing off describes' someone's voice:
"Susan started telling John about the weekly sales report, then her voice trailed off as she looked over his shoulder."
Either of these are acceptable, although I think the latter (where the voice trails off, not the person) may be more correct.
When referring to typed or written text, however, i don't think that this is quite right without the addition of the ellipsis (...) which indicates that the trailing off is a deliberate part of the sentence. For example, this is trailling off:
"She hasn't been here for months. In fact, now I think about it..."
which tells the reader that the speaker has trailed off.
If it was just this:
"She hasn't been here for months. In fact, now I think about it"
It just looks like a mistake. However, you could, i think still call it "trailing off", eg:
"Lately, I suffer from an unusual habit. I trail off in the middle of sentences, while typing, and I don't even notice it."
and I think people would understand what you meant.
You could say that your sentences are truncated:
shorten (something) by cutting off the top or the end.
"a truncated cone shape"
synonyms: shorten, cut, cut short, curtail, bring to an untimely end; abbreviate, condense, reduce, prune
"the program may need to be truncated"
"Sorry, I noticed that I've been truncating sentences in my emails."
You might also combine the answers and say something like "I've been trailing off in my emails and truncating sentences."
This rhetorical device is called aposiopesis.
Aposiopesis (/ˌæpəsaɪ.əˈpiːsɪs/; Classical Greek: ἀποσιώπησις, "becoming silent") is a figure of speech wherein a sentence is deliberately broken off and left unfinished, the ending to be supplied by the imagination, giving an impression of unwillingness or inability to continue.