From what you've said, I think you have a lot to be proud of. You've pursued education for three years now and that on its own is a huge amount of effort and time. To still be going at it is amazing. That, and the fact that you're doing all you can to remain independent (and it's working).
The fact that the salaries aren't meeting your responsibilities is definitely going to be soul-crushing, but again you're in the process of looking for something better and that's what's most important right now: finding something that actually makes you happy and provides enough stability for you to survive.
I have been trying to get a new job in the past few months, had some great interviews, but at the end there was always someone else that they liked more.
I really had high hopes in 3 of them, but the response was always negative.
I've been in similar situations in the past, years back I was doing a lot of freelance writing and would get really close to getting full-time positions and publications, but at the last moment and after endless annoying little tests they'd just say "Well, we found someone better." and that crushed me. It was such an insanely competitive industry that paid so little but demanded so much work from you. Very parasitic. Eventually I just gave it all up, I stopped writing work entirely. Though it led to me enjoying my time on here and finding filmmaking.
what am I doing wrong? why I can't get something better?
I hate that, I hate that so much. It eats away at your mind. Constantly, at the worst times, too. I was recently experiencing similar thoughts due to the fact that the person I loved was effectively cheating on me with someone I considered a friend. I was dropped instantly and left with no answers, which only led to those very same questions: what did I do wrong? Is it me that isn't good enough? What must I do to be better?
A lot of the time, the truth is you aren't really doing anything wrong. You can always be better, sure. Nobody is perfect and life is a process in which we constantly learn as we age and experience. But to think you are doing something wrong or not actually good enough is the worst mindset to be in. Instead, you have to play your mind and bend it into thinking differently; instead, think of how you could go the extra mile to stand out. What could you do differently to others? Note that I said differently, not what could you to better. You just have to find ways to stand out and really promote yourself.
I think I will take a look at my mindset, always going for the bigger prize as soon as I achieved the current one without enjoying it; also realized that I'm comparing others with myself too much and forgot the fact that I'm my own person.
These are some really great advices and I will try my best to follow them. Thanks for the support!