Hey SyndullaRen24!
First, a mandatory disclaimer regarding tattoos in Elvish: Our understanding of the Elvish languages is constantly evolving with new research and new publications, and even the best efforts of an expert today could become outdated and sometimes prove altogether false in a couple of years. There are also many contentious topics where the experts don't agree with each other. As such, I would strongly discourage you from getting tattoos in Elvish. It is much safer to transliterate English words into Tengwar.
Some comments on your translation:
I'll start with "You can", since that's easier. The verb stem here is pol-, which indeed implies physical capability. The aorist tense uses i as the vowel between the stem and the pronominal suffix, so "you can" = politye. Note that the diaeresis on the final e is optional in the Latin orthography, and altogether ignored in Tengwar; if you do use the diaeresis, it only ever goes on a word-final e or when you have two vowels in a sequence (which are not a diphthong).
Similarly, "I can" would be polinye. So far I'd say things are safe enough even for tattoos, but... Things get very uncertain when you add the negation, and this is where I will once again recommend to go for a transliteration instead.
The thing is, Tolkien vacillated between two different negation concepts for most of his life, and it is not really clear that he ever settled on anything. What you used here is "LĀ negation", which I personally favour, mostly because of a very extensive late document regarding its use. However, after he wrote that document he once again changed his mind and wrote "back to ú" (the other negation concept, "Ū negation"); consequently many experts favour Ū negation instead. There is no consensus!
Even if there was consensus on whether to use LĀ or Ū, there's another point on which experts disagree: whether the negator is a particle or a verb. To give an example using LĀ and past tense, "I couldn't" can be rendered as lá pollenye, láne polinye, or lánye pole. In the aorist it's easier, it's just a question of where the subject suffix appears - lá polinye vs. lánye pole. If you ask me, both options could exist at different historical stages of the language, with the latter being more archaic.
Regarding "you promised" - bear in mind that Tolkien actually deleted vesta- "to swear", and instead this verb was at one point used for "to wed". I'm not sure what other alternatives to offer though. Evestanietye is correct as the perfect form.
I hope this helps!
