You are eligible for the first 4 years of post-secondary education, if you were enrolled at least half-time. As far as whether you are eligible as a 5th year senior, the IRS has this to say:
A student has completed the first 4 years of postsecondary education if the institution at which the student is enrolled awards the student 4 years of academic credit at that institution for coursework completed by the student before 2020. This student generally wouldn't be an eligible student for purposes of the American opportunity credit.
If you were enrolled more than half-time for any 4 years of the 9 year history you talk about, and you got credit, then you aren't eligible any longer. If you were enrolled half-time or less, those years don't count against you for the AOTC.
Potentially, if you were enrolled more than half-time in 5 different years but didn't get full credit, you might be eligible since you never claimed it before, but I would want a second opinion on this.
Yes, you could also amend your 2018 and 2019 returns if you are eligible. Whether you get any money depends on the factors @Hal_Al discusses. You generally have to have income from working and owe income taxes, or be age 24 or older to get any part of the credit.
Lastly, if you did not file a 2020 tax return, and you can't be claimed as a dependent by someone else, you may be eligible for a recovery rebate (stimulus) on your tax return, separate from the AOTC. If you already got your stimulus checks, you won't get extra by filing.
