I can definitely relate to this.
Even though I have two degrees, three internships, and two certifications, the last time I got a raise I felt like I didn't deserve it, it was too much and soon they would figure out that I wasn't worth it. I went through a whole period where I was struggling with imposter syndrome at work. Every time I would get promoted, I would feel like I had to work that much harder to prove that I was worth the promotion. But it's silly because they didn't promote me based solely on what they thought I would do in the future, they promoted me based on what I had done in the past and how I had proven to be worthy.
I had the same problem in school. Whenever I would finish a year of school, even if I got straight As, I would think to myself, this is temporary, this year was just unusually easy and next year the real work begins. It will be twice as hard, three times as hard, four times as hard and you will fail. I convinced myself that the level of difficulty in life just always, constantly increases and eventually I would reach my maximum ability.
Well, it's been almost 15 years since I entered the professional world, and my predictions/self-sabotage haven't come true. I think for me, it had a lot to do with my fears of what other people are thinking and that I can't trust them. I felt that people only said nice things and promoted me because they were nice, and it wasn't a reflection of my abilities. I didn't trust them to be honest with me about whether or not I was a good performer. If I got a good performance review, I thought it was because the manager just didn't have the guts to give me real, candid feedback.
But after going to support group for awhile and getting therapy, my eyes started to open to the things I was saying to myself, the voices in my head. I started to notice more that I was actually capable, and sometimes more capable than those around me, and that's why I got promoted or got raises. I'm still afraid of failing, afraid that it's all a house of cards and I'll knock it down somehow. But at least, like 60% of the time, I can suppress those voices.