I'm trying to wear Hijab but it's forbidden in Tunisia so I need some advice to know if I really have to take it off.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We supplicate Allah to support the questioner sister in this trial, reward her good for her persistence and make her steadfast on the right path.
The answer to your question is that if you can migrate from this country to another country where you can freely follow the religion then you should do so, to protect your religion, faith and honor. If you cannot migrate then you have to act according to the following details:
If the harm could affect any part of your body or honor or mind then it is permitted for you to unveil to avoid such harms. This ruling is based on an established fact in Sharia that necessities know no rules.
However, you should take into consideration the following principles:
1) You should not go outside except for a necessity.
2) If there is no necessity you should stay in your house to avoid the sin of unveiling.
3) The unveiling should be only in the limits that prevent you from the suspected harms.
Know if the harm that could affect your body, honor or mind is not so serious then you should not unveil yourself. There is a great reward in bearing the hardship for following Allah's commands. Allah Says (interpretation of meaning): {So their Lord accepted of them (their supplication and answered them), "Never will I allow to be lost the work of any of you, be he male or female. You are (members) one of another, so those who emigrated and were driven out from their homes, and suffered harm in My Cause, and who fought, and were killed (in My Cause), verily, I will remit from them their evil deeds and admit them into Gardens under which rivers flow (in Paradise); a reward from Allâh, and with Allâh is the best of rewards."} [3: 195].
Allah knows best.