You're doing well, each experience is useful and I am sure you'll get more comfortable with it the more you do it. It's very impressive you've got this many interviews/assessment centres, you must be very good! Don't give up. It's a difficult market at the moment unfortunately.
I've always been a bit iffy about these interviews or centres where they're judging you as a person, because everyone expresses differently and I feel it can sometimes be a bit discriminatory... By the sounds of it though, you're not bad either in behaviour or exercise if your feedback is that split, the assessment side of things is easy to fix as it just means more studying but if you're wanting to figure out how to tackle the behaviour side, might I suggest looking into interview coaching or practise (with friends/family)? My father suggested it to me as my ND traits are hindering my performance a bit and I think it is a good idea. They can give you more detailed feedback about how you should better present yourself and it should get you more comfortable in these situations.
Keep trying, you'll get there! 🙂 but if you can, take some time to recover a bit, I understand these things can burn one out.