How do you say ‘You will heal’ in Latin? I mean this in a way of being able to mentally heal.
"Heal" in English is what's called an "ergative verb". When it's used with a single noun, that noun is the person who gets better; when it's used with two nouns, the subject is the doctor, the object is the person who gets better.
The relevant verbs in Latin unfortunately don't have that property: if it's used with a single noun, that noun is the doctor. So a literal translation of "you will heal" would be more like "you will make people better".
So instead, I would use a passive verb: literally, "you will be healed". Passive verbs in Latin also sometimes have a "middle" meaning, something like "you will heal yourself".
Conveniently, this is a single word in Latin: sānāberis for a single person, sānābiminī for multiple people.
If you want to express what exactly will be good for their health, you can add a noun in the ablative. If you want to say they will be healed by time, for example (because time heals all wounds), add the word tempore; if you want to say they will be healed by therapy, add the word therapīā, and so on.
Celsus (c. 25 BC – c. 50 AD) often uses the intransitive verb sanescere in his De Medicina to express the non-causative change of state meaning.
Insanientes sub somno sanescunt (Cels. 3, 18).
Unlike sanari, which is in principle ambiguous between a passive interpretation (involving an (implicit) agent/cause: "by someone/something") and a middle one (cf. Draconis's answer), notice that sanescere is not ambiguous. So sanesces/sanescetis is a better translation for 'You will heal' if an internal process is involved.
As for the use of adjuncts pointed out by Draconis, here are my choices/preferences: tempore is compatible with both sanari and sanescere, whereas therapia is probably better with sanari (NB: only the latter involves an external cause).