I often find myself saying things like "I have to finish at the end of a chapter because I'm OCD". Here, I'm just using the term colloquially. I dislike using it because I don't have OCD, and I'd like to not be subtly perpetuating the misconception that real OCD is just a minor or very mild thing. I considered going with "obsessive compulsive", but that strikes me as long-winded and too likely to make the person default to thinking "OCD". Any suggestions?
If you have to finish the chapter because you're obsessed with the material, you could use obsessed, or obsessive.
If you have to finish the chapter because you feel compelled to finish what you've started, you could use compelled or compulsive.
It's probably going to be better to recast the sentence to use compelled ("I feel compelled" or similar) or obsessed (as with compelled) than to use either obsessive or compulsive.
A technical term that has been overused by laymen approximating the technical sense, in much the same way that 'obsessive' and 'compulsive' have been so abused, is 'fixated'.
Another approach, but one which guesses at your reasons for feeling compelled to finish the chapter, is to use 'systematic' or 'highly organized' or another variant of those: 'compulsively systematic', 'obsessively systematic', etc.
[Personally, I don't have to finish the chapter, but usually want to because doing so helps me remember where I was without using a bookmark which I have somehow lost track of. It also helps me remember the preceding material. All of that I can do without, but prefer not to.]
In some contexts, though perhaps not the one in your example, you could say that you were a 'perfectionist'. It would describe someone who could not leave a task uncompleted or who would carry out a task with excess care. Perhaps someone who would clean their kitchen cupboards with a toothbrush and not go out in the evening because they hadn't finished it!
Perhaps one can use the term paranoid in its non-medical sense. Using paranoid with about usually implies the non-medical sense; that the subject is just being too obsessive/anxious about a particular thing/topic.
"I have to finish at the end of a chapter because I'm, well, paranoid about it".
1.1 Unreasonably or obsessively anxious, suspicious, or mistrustful:
‘you think I'm paranoid but I tell you there is something going on’
‘I am paranoid about personal hygiene, and I constantly wash clothes and burn candles in my apartment for company.’
‘‘People are paranoid about what they can eat and what they can't,’ Jenny points out.’