To Brood
To Think Hard, To Worry
transitive verb, ichidan verb
詰める is "to stuff in," so if you're really stuffing in your thoughts (think), you're really trying to think hard. That's just another way of saying to brood.
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.