To Brood
To Dwell On, To Take Something To Heart
transitive verb, ichidan verb
詰める is "to stuff in." If you're really stuffing in your thoughts (think), you're thinking very deeply and seriously. So this is to brood, to dwell on, or to take something to heart.
思い詰める suggests you're pondering deeply over something serious or distressing, bordering on overthinking.
The reading is 思い and 詰める put together into one reading.
私は失業中ですが、そのことを思い詰めてはいません。
I’m between jobs, but I’m not worrying too much about it.