1. I can´t trust you more after this incident. 2. Don't throw away the possibility to (of being able to travel to another country-could be said instead) travel to another country. 3. It's so hard to get rid of this habit. 4. It was the final straw. I go away(? Depends on the context but i'll go away seems correct). 5. It's useless to threaten me, I'm not afraid of you. 6. She is already a difficult child to deal with/to handle. I have made the (come to the)decision (that)she will (be) study in a boarding school. 7. I thought his visit to the police station would straighten him out. 8. They made a swear(vow) never to leave each other. 9. They don't tend to their children.

check, please

1. I can´t trust you anymore after this incident.
2. Don't throw away the possibility to travel internationally.
3. It's so hard to break this habit.
4. It was the final straw. I left.
5. It's useless to threaten me, I'm not afraid of you.
6. She is already a difficult child. I have made the decision for her to  study in a boarding school.
7. I thought his visit to the police station would help him.
8. They have made us swear  never to leave each other.
9. They don't take care of their children.

check, please

1. I can´t trust you any more after this incident.
2. Don't throw away the possibility/chance/opportunity to travel to another country.
3. It's so hard to get rid of this habit.
4. That was a final straw. I go away.
5. It's useless to threaten me, I'm not afraid of you.
6. She is already a difficult child. I have made the decision that she will study in a boarding school.
7. I thought (that) his visit to the police station would straighten him out.
8. They have made an oath never to leave each other.
9. They don't tend to their children.

(Strongly optional words here). With think, say, find, assume, suppose, ... that ... one often omits 'that', but I wouldn't.

Very well written!