If you want to say "feel everything" in an advise-wise sense, is "sentire omnia" the correct way to say it? Or maybe it will be better to say "Sentias onmia"?
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Sentire is an infinitive, corresponding to the English "to" form. So sentire omnia by itself means "to feel everything" (depending on context, one might translate it differently). It is not how you would formulate an advice.
Sentias omnia (may you feel everything) is better, but in my opinion sounds more like a wish. You could perhaps use the passive subjunctive: Omnia sentiantur (let all be sensed; cf. audiatur et altera pars, let the other side also be heard). But that is also a wish. A wish that's stronger becomes a command, not an advice.
In my opinion the best choice is the imperative. Despite its name, the imperative is not exclusively or even predominantly used to issue commands, but also to utter requests and indeed advice.
See, for example, this passage from Cicero (1st Catlinarian oration) where he gives Catilina advice (it is ironic advice, since Catilina is his bitter enemy, but still advice and no request or command):
Sin autem servire meae laudi et gloriae mavis, egredere cum inportuna sceleratorum manu, confer te ad Manlium, concita perditos cives, secerne te a bonis, infer patriae bellum, exsulta impio latrocinio [...]
But if you would rather advance my fame and glory, then leave [Rome] together with your savage band of miscreants, go to Manlius, rouse the rabble, separate yourself from the patriots, bring war upon your hometown, revel in your godless villainy ...
The imperative would look like this: