At this stage in my life, I truly believe describing my lady as my “partner” says it all. To say “companion” is much friendlier, but somewhat akin to the synonyms “consort” or “ally”. There is that faint suggestion of inequality; somewhat mindful of someone that simply follows. For example, saying “We’re partners” flows easily, and is expressly sincere, albeit lacking in emotional intimacy; it is rather bland descriptively. More likely than not, that’s the reason why a descriptively decorative adjective is necessary, e.g. “life” partner. Unfortunately, at least at this point in time, that particular phrase is not universally applicable to all relationships.
“Intimate partner” is probably as close to the truth as you can get. In reality, however, that is just way too much information to be shared publicly. Use that phrase openly and you might just have to add the qualifier “previously” in front of “intimate” for an indefinite period of purgatorial confinement.
“My companion” at first sounds great but when it’s “We’re companions”, ouch. Reminds me of devoted pets. There is an essence of possession and self investment-probably why “my companion” works better than “we’re companions”.
“My girlfriend” implies a superficiality and transience. Fun, yet readily disposable as there’s no real investment to intimacy.
“My significant other“ as a term of endearment is simply horrid. Whoever it was that coined that phrase had absolutely no respect for for their “special person” and probably only considered them as an attachment to be displayed or hidden depending on the situation. Might as well say, “Oh honey, I love you so much! You are so convenient!”.