What's real and what's my mental illness

I've been in my current relationship for over a year and a half and we are now living together (since October). Pretty much the whole time we've been in a relationship I've been in the headspace that it won't last, that I'm meant to be alone, and anytime we disagree or argue I automatically panic we'll break up. I can't tell if all these thoughts are because I actually want to break up or if it's my severe anxiety and worry and assumption that things can't last. I've had one other serious relationship 9 years ago and I was a similar way in that one too. I struggle to make decisions and am uncomfortable in my skin and my decisions, so I can't tell if I'm unhappy in our relationship, unhappy in any relationship, and/or its all part of the fact that my mental illnesses interfere with everything in my life. Which makes me feel like I'm a piece of shit, a bad partner, a burden - or if we are actually not meant to be. How can I know what's real, what to actually worry about or what I'm overreacting about. I don't know.