Oh please don't do this to yourself. I hate when this happens. Imposter syndrome at it's finest.
Over the years, I've had several staff come to me in a similar position to you, doubting themselves purely because they don't have a degree or certain qualifications. I can give them good feedback, excellent appraisals, salary reviews etc but they really struggle to take it on board just because of no University.
University or degrees is only one form of intelligence and is proves fuck all other than people can pass exams. Some highly qualified people have no ability to apply anything from university anyway, they simple just have good memories & exam technique. It often doesn't translate into decent at their job.
You got your role on merit.
Someone thought you were good enough for the role.
Your feedback is excellent.
People without degrees place far more importance than those who don't have them.
Do you really think that someone would have hired you for an reason other than they saw something in you that made them certain your at the right choice?
If you don't have faith in yourself at the moment. Can you look at the person who hired you? Do you rate them? Do you think they make charitable or stupid decisions or they were smart enough to hire you because they know you are up to the job?
This breaks my heart sometimes at work to the point I've ended up asking staff if they think I'm lying to them or something?
PM me or ask me anything that you think might help but please believe me.
I have lost good staff over this sort of thinking so I feel very strongly about it.