How do i know if i have impostor sydrome or if i actually am bad at my job?

I feel really shitty at my job most of the time, and recently i also heard that a colleague said about me behind my back that i am dumb after i asked him a few times when i wasnt sure about something (i often get tunnel vision and panic when i'm unsure and then need to ask somoebody else).

I mean, my feelings of inadequacy, worthlessness and uselessness lead to me seeming very insecure, sometimes my mind feels empty when i feel stressed or pressured, and when i am not sure about something i tend to say "i dont know", while i think most of my colleagues also just decide something even when they arent 100% sure - so i get how people might think about me that i'm stupid or slow (plus i work only part time and often am on sick leave for mental health reasons, that probably also doesnt make me popular).

But still... how tf am i supposed to know if i have an impostor syndrome or really am one if i dont have many moments of success? The one thing that makes me think i'm at least not a total failure is that my contract has been prolonged but that might not mean a lot cause nobody is ever fired here unless they really fuck up. But as long as i think i suck, my mental health wont get much better in the foreseeable future which leads to more sick leave . I am already in therapy but i have so many issues that this is not top priority right now.