Although are/were are used to form questions, they don't work with met. It should be have you met [her] before? (If these two people are both present, her is not necessary; you only need it if you're talking about someone who's not there at the time).
In questions that use subject auxiliary inversion (e.g. when You are ... becomes Are you ...?), and that don't use interrogatives (e.g. who/what/where/etc.), there is an easy way to tell whether or not you're using the correct auxiliary: simply rearrange the sentence to see if it makes sense as a statement.
For example:
Are you met before? You are met before.
-versus-
Have you met before? You have met before.
The second is the only one that makes sense. That's the easiest way to figure it out.
You do have one other option here; you could use do as the auxiliary. But in this case, it can't be used with met. You'd have to switch the verb to know for that to work. And it would need to be in the present tense. The rule that I've mentioned above still applies:
Do you met her? You do met her.
-versus-
Do you know her? You do know her.
You'll know to use know because it agrees with do in tense (i.e. both simple present).
Most often, these types of questions use have, had, or do.
Questions involving forms of to be (e.g. are/were), are generally about states of being/existential, mood/feelings/emotional states, location, and actions about to occur: (e.g. Are we going to leave now?, Are you there?, Are you mad?, Were they at the party?, etc.).
Depending on the context and temporal factors, you might be able to use did. This would make the main verb present tense though. If this is after the fact, you could instead say:
Did you meet her? You did meet her.
Did you know her [back then]? You did know her [back then].
This a rather broad topic coming under the general description of collocations (Def. placement). There are verb-noun, verb-adverb,verb-verb, etc. They are idiomatic and there are no real rules, only guidelines in some cases.
In this case, it should be "fulfill (BrE fulfil) a promise" and fulfill or meet a "commitment" (OED) http://oxforddictionary.so8848.com/search?word=commitment.
Some people will disagree on these, but most native speakers just know them intuitively: they sound right.
You can also look in Google Dictionary,
as well as: http://prowritingaid.com/en/Collocation/Dictionary?word=fulfil
Best Answer
In this sentence, you don't actually have the past tense. You have the Present Perfect here. The Present Perfect is formed in the following way: the verb 'to have' in the Present Simple—which is either 'have' or 'has'—plus the 3rd form of the verb (Past Participle). So the word 'met' in 'Have we ever met?' is a Past Participle form and not past tense.
Here we have another English tense, the Past Simple. But please notice that we have an interrogative sentence. More exactly, we have a yes/no question. In English, interrogative sentences in the Past Simple (excluding questions where the wh- word is the subject) are constructed using the auxiliary verb 'do' in the Past tense (that is, 'did') plus the infinitive of the main verb without 'to'. The subject is usually placed between the auxiliary and the main verb. You cannot use the 2nd form, the Past Simple of the main verb. Thus, you use 'meet', which is infinitive without 'to', and not 'met'.
A few words about usage of these sentences. Personally I consider the first one is more correct. We have the word 'ever', so I would use Present Perfect here ('Have we ever met?'). I'm not comfortable with the Past Simple. But maybe in American English you could use it as well.