Both, and that is not vagueness. Thai locates thinking and feeling in the same organ, and it builds a large part of its emotional vocabulary by compounding ใจ with something else.
Once you can see the pattern, several dozen words stop being separate items and become two morphemes each.
Two shapes, and the order flips the meaning
Something + ใจ describes the state of the heart, usually passing. ดีใจ is good-heart: glad, right now. เสียใจ is spoiled-heart: sorry, upset. ตกใจ is fallen-heart: startled.
ใจ + something describes the character of the heart, usually permanent. ใจดี is heart-good: a kind person. ใจร้อน is heart-hot: an impatient one.
ดีใจ and ใจดี are the same two morphemes in opposite orders, and they are a temporary feeling and a lifelong trait. That is the pattern in one pair.
- ดีใจdiː.tɕaj
glad
good + heart: a state
- ใจดีtɕaj.diː
kind
heart + good: a character
- เสียใจsǐːa.tɕaj
sorry; upset
- ตกใจtòk.tɕaj
startled
- ใจร้อนtɕaj.rɔ́ːn
impatient
Recognise, do not invent
The compounds are transparent enough to remember and not transparent enough to derive. เกรงใจ from week 3 is not recoverable from เกรง and ใจ if nobody has told you. น้อยใจ is not 'slightly hearted'.
So use the pattern for storage and not for production. When a compound does not decompose, learn it whole and stop parsing. That is the honest instruction for this whole week.
- เกรงใจkreːŋ.tɕaj
reluctant to impose
not derivable, and one of the most important words here
- น้อยใจnɔ́ːj.tɕaj
to feel slighted
The ones you already had
เข้าใจ is enter-heart: to understand. ตั้งใจ is set-heart: to intend, to concentrate. สนใจ is to be interested.
You have been using these since week 2 without knowing they belonged to a family. That is the ordinary experience of this pattern: it is already in your vocabulary, and this week is only pointing at it.
- เข้าใจkʰâw.tɕaj
to understand
- ตั้งใจtâŋ.tɕaj
to intend; to concentrate
- สนใจsǒn.tɕaj
to be interested