Depuis que je fais de la méditation, mon humeur est plus stable.

In French, when an action started in the past and is still true or ongoing now, you normally use the present tense with depuis or depuis que.

• Depuis que je fais de la méditation
  = Since I’ve been doing meditation / Since I started meditating

English often uses present perfect (have been doing), but French keeps the present:

• Je fais de la méditation depuis deux ans.
  = I’ve been meditating for two years.

Using j’ai fait de la méditation depuis… is usually incorrect in standard French for this meaning; it would sound like the action is completed, which conflicts with depuis (which implies continuity up to now).

• depuis = since / for, followed by a noun or a time expression

  • Je fais de la méditation depuis 2020.
    I’ve been meditating since 2020.
  • Je fais de la méditation depuis deux ans.
    I’ve been meditating for two years.

• depuis que = since, followed by a clause (a subject and a verb)

  • Depuis que je fais de la méditation, mon humeur est plus stable.
    Since I’ve been meditating, my mood is more stable.

So:

• depuis + noun / time expression
• depuis que + subject + verb

Both are grammatically correct, but they’re not used in exactly the same way.

• Je fais de la méditation
  Literally: I do (some) meditation.

  • Very common, neutral, and idiomatic.
  • Follows the pattern faire de + activity:
    faire du sport, faire de la natation, faire de la musique, etc.

• Je médite
  Literally: I meditate.

  • Also correct, a bit more direct and perhaps a little more formal or “literary” in some contexts.
  • In everyday speech, many speakers prefer faire de la méditation.

In your sentence, je fais de la méditation sounds completely natural and is probably the more common choice in conversation.

De la here is a partitive article, used for an uncountable activity or substance, roughly like some in English.

• Je fais de la méditation.
  ≈ I do (some) meditation / I meditate.

We use:

• de la before a feminine singular noun (like méditation) for an undefined quantity of something:
  • Je bois de la soupe. – I drink (some) soup.
  • Je fais de la musique. – I make (some) music.

La méditation (the meditation) would sound like you’re referring to meditation in general as a concept:

• La méditation est bonne pour la santé.
  Meditation is good for your health.

Des méditations (plural) is possible but would mean separate, countable instances of meditation sessions, and it’s not the form used in this kind of general statement about a habit.

You’re right: humeur is a feminine noun.

Normally:

• ma = my (before feminine singular nouns)
• mon = my (before masculine singular nouns)

However, French switches from ma to mon before feminine nouns that start with a vowel or a mute h, to make pronunciation smoother:

• mon humeur (not ma humeur)
• mon habitude (from une habitude)
• mon image (from une image)

So:

• humeur is feminine, but it begins with a mute h, and we use mon to avoid the awkward sound of ma humeur.

Literally:

• mon humeur est plus stable
  = my mood is more stable.

This is a perfectly natural and common way to express this idea in French. The structure is:

• mon humeur – my mood
• est – is
• plus stable – more stable

You could optionally add qu’avant (than before) for emphasis, but it’s not required:

• Depuis que je fais de la méditation, mon humeur est plus stable qu’avant.
  Since I’ve been meditating, my mood is more stable than before.

In French, plus is used in two main ways:

1. plus + adjective/adverb (no de)

   • plus stable – more stable
   • plus rapide – faster
   • plus souvent – more often

2. plus de + noun

   • plus de patience – more patience
   • plus de temps – more time
   • plus de problèmes – more problems

In your sentence, stable is an adjective, so we use plus stable with no de:

• mon humeur est plus stable – my mood is more stable.

Plus de stable would be incorrect here.

You can say that, but it changes the nuance, and it’s less natural in everyday French.

• mon humeur est plus stable (present)
  → Describes a current state that is considered true now, and generally sounds more natural after depuis que.

• mon humeur a été plus stable (passé composé)
  → Feels a bit more like you’re looking back over a period as a kind of “completed” span:
  My mood has been more stable (over that time).

With depuis que, French strongly prefers the present to emphasize that the situation started in the past and is still true now:

• Depuis que je fais de la méditation, mon humeur est plus stable.
  is the default, most idiomatic version.

Yes, a very common alternative pattern is ça fait… que (or il y a… que in some regions), which also expresses “since / for” with duration:

• Ça fait deux ans que je fais de la méditation et mon humeur est plus stable.
  I’ve been meditating for two years and my mood is more stable.

But to keep exactly the “since I started meditating” idea with a cause–effect feel, depuis que is perfect and very natural.

If you want to mirror the English idea of “has become more stable,” you could say:

• Depuis que je fais de la méditation, mon humeur est devenue plus stable.

Here:

• est devenue is the passé composé of devenir (to become).
• This emphasizes the change that has happened, whereas mon humeur est plus stable simply states the current result.

Both are correct; the original sentence is slightly simpler and more commonly used in everyday speech.