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It seems what you really want is the phrase ‘Master thyself to conquer thyself’, so I will have a punt at that.
Grammar and word choice
The suffix ‑met¹ is very useful: It points back at the noun or pronoun and intensifies it, so tēmet fits the bill. As Joonas points out, another good option is to use ipsum: Pointing back to what was before, tē […] ipsum would translate well as idiomatic ‘yourself’, more literal as ‘you yourself’. I will choose the prior, though, as a nod to tēmet nōsce.
For ‘master’, one option is domō (1), meaning to tame or break in, or to subdue, conquer, vanquish.¹ ² I do not find this a good option. A better option might be teneō (2), which carries numerous meanings: hold, keep; comprehend; possess; master; preserve.¹ There is an idea of restraint in it, which its etymology reveals:
From Proto-Italic *tenēō, stative from Proto-Indo-European *ten- (“to stretch, draw”), see also Ancient Greek τείνω (teínō), Persian تنیدن (tanidan, “to web”), Sanskrit तनोति (tanóti) and Old English þennan.
The 2nd person present active imperative of teneō is tenē.
For the idea of ‘to conquer thyself’ I suggest the conjunctive as a means to express purpose:
The subjunctive in the clause of purpose is hortatory in origin, coming through a kind of indirect discourse construction (for which see § 592). Thus, mīsit lēgātōs quī dīcerent means he sent ambassadors who should say, i.e. who were directed to say; in the direct orders the verb would be dīcite, which would become dīcant in the indirect discourse of narrative (§ 588) or dīcerent in the past (cf. hortatory subjunctive in past tenses, § 439.b). The subjunctive with ut and nē is, in general, similar in origin.
A clause expressing purpose is called a Final Clause.
Final Clauses take the subjunctive introduced by ut (utī), negative nē (ut nē), or by a relative pronoun or adverb.
Allen and Greenough Latin Grammar
I think a good option for ‘to conquer’, would be superō (1), in the meaning of to I ‘surmount, rise over, rise above, go over, ascend, overtop, mount’ or to be ‘superior, surpass, exceed, excel, outdo, outstrip, be in excess, have the upper hand’.¹ ² The 2nd person singular present active subjunctive of superō is superēs.
Another option would be ēvincō (3: ēvincere, ēvīcī, ēvictum), which carrie the meanings of to ‘overcome, conquer, subdue, overwhelm, defeat utterly; prevail, bring to pass; demonstrate, show, evince; persuade; evict’.¹ ² The 2nd person singular present active subjunctive of ēvincō is ēvincās.
For this part of the expression, I believe tē ipsum would be an excellent choice.
I suggest two option:
- Latin: Tēmet tenē ut tē ipsum superēs.
- English literal and grammatical: Master yourself for the purpose that you will have the upper hand on you yourself.
- English idiomatic: Master yourself to surpass yourself.
- Latin: Tēmet tenē ut tē ipsum ēvincās.
- English literal and grammatical:
- Master yourself for the purpose that you will conquer yourself.
- Master yourself for the purpose that you wlil rise above yourself.
- English idiomatic: Master yourself to overcome yourself.
You could also, perhaps go for a khiasmic construction, either ut tē superēs ipsum or ut tē ēvincās ipsum; doing that, you stress the verb a tad bit more; also it has a nice touch to it, not unfamiliar to the ancient writers.
As always, should anyone have any feedback to this (I am far from at the height of masters such as Asteroides, cmw, Joonas Ilmavirta, and numerous others here – but I am learning!), I would be most happy to improve on this answer.