grammar - Using a present tense verb in an "if" clause, followed by the main clause

It is certainly not true that you must always follow the prototypical second conditional format, namely past tense in the if-clause and would + infinitive in the main clause.

It is worth quoting Michael Lewis in The English Verb (p148) to see why this is the case:

It is the verb phrase not the sentence which is the fundamental unit
requiring analysis. Certain combinations are, for semantic reasons,
highly frequent, while others are less frequent or even impossible. ...

A particular misunderstanding frequently arises in the teaching of
so-called conditional sentences. It is common to teach three basic
kinds. ...

If students are taught only the first, second and third conditionals,
they will know only a small, admittedly highly frequent, sub-set of
the possibilities. It is not necessary to teach the fourth
conditional, the fifth conditional, etc., but it is important to
recognise that the possibility arises from the meaning of the
individual clauses ... . The explanation of the use of a form in a
conditional sentence is exactly the same as that of its occurrence in
any other utterance. The underlying principle behind this is that each
main verb phrase is treated independently.

So on this basis we can speculate that the present tense is used in If you resign now, because the 'you' may indeed be seriously thinking of resigning. Using the past tense If you resigned now could imply that the speaker feels this is a more remote possibility. Similarly, the use of will in the main clause (you will feel sorry later) makes it a stronger prediction than you'd feel sorry later.

There are many Google hits for this combination of tenses with the string "if you do that you would". For example:

If you do that, you would be surprised how competitive we could be.

If you do that, you would have no problems — but if you shut the car
off with the radio on, it would draw the battery down over a week's
time.

If you do that, you would be caught-up in the chicken and egg
question.

If you do that, you would exceed your standard quota of three felonies
a day.

If you do that you would not able to use ffmpeg in other software.

And if you do that, you would have enough to pay for something better.

If you do that you would be compromising with reality.