As a goy, can I conceal a vow ? I read it’s an obligation to fullfil (Mitzvah Sichlis), and I’m not really sure anymore about what I swore (and I didn’t know it was a sin. now I would swar nothing)

Shalom, Thank you for your question. On our hebrew site the esteemed Rabbi Yakov Ariel writes that a vow made by a non-Jew has the force of a vow, and the non-Jew must be careful to fulfill their vows and promises. He sites the fact that our forefathers engaged non-Jews in oaths and vows. With this on mind you are correct that you should be careful not to take on any vows or oaths. (Religious Jews have a practice of adding the words "Bli Neder" – that is, "without taking on a vow" whenever they make a statement that could be construed as a vow). As to the past – if you do not have any clear recollection of an exact oath or vow you took, then my advice is to focus only on being careful in the future. You were unaware of the importance of the subject at the time, as well as the fact that you may well not have taken an actual vow (there are various technical laws about the language and intent of vows that you may well have managed to avoid). May you have many blessings.