Goodness.
Sacraments are valid unless proven otherwise by competent authority. Not random internet people.
A marriage is valid until a tribunal issues a decree of nullity. Someone opining on the internet that one has grounds for annulment does not nullify the marriage.
You aren’t going to hell because some random person thinks your confession wasn’t valid. For one thing, perhaps there was a misunderstanding on the part of the priest or the pentitant. Perhaps we are not hearing the whole story. Assume it’s valid. If it troubles you, mention it next time.
No you are wrong and this is bad advice. I have proven my position and affirmed by Fr. Edward. The OP was clear enough: she did not confess anything, and therefore had no matter for the sacrament, rendering it invalid.
Your attempt to liken it to the presumption of validity of marriage is flawed. For one thing, the law states that MARRIAGE enjoys the presumption of validity. It’s not sacraments in general. And then, only when defects and impediments are occult. It does not take a tribunal to know, for example, that a Catholic married outside the Church does not enjoy the presumption of validity. A priest saying the Words of Consecration over a rice cracker does not enjoy the presumption of validity. Ordaining a woman does not enjoy the presumption of validity. Because these defects are manifest: the is a defect of form or matter, which renders any sacrament invalid. This is the case there: there was a manifest defect, which means there are no grounds to presume validity.
It does not take more than a catechized Catholic to know that without the essential matter, there is no sacrament. That is why at least one sin must be mentioned in confession and if one has no sins to confess (as in a devotional confession), he must mention at least one previously forgiven sin to receive absolution, otherwise, the priest has nothing to absolve. One does not need a tribunal to know that.
And as for “random persons” this should be a lesson to the OP, as to any other scrupulous, that asking questions here is a bad idea. But now that she has opened that can of worms, the truth, whether from “random strangers” or from a priest is still the truth. In this case, she has it, painful as it may be. She was cheated by that priest, but now hse has the knowledge to rectify it. And “assuming it’s valid” is not the next step. Going to valid confession is.