Ahora me siento más seguro cuando hablo en clase.

Because sentirse is a reflexive verb when it means to feel in the sense of how someone feels.

• me siento = I feel
• te sientes = you feel
• se siente = he/she feels

If you say just siento, it usually means I feel in the sense of I perceive or I regret:

• Siento frío = I feel cold
• Siento que... = I feel / I think that...
• Lo siento = I’m sorry

So in this sentence, me siento más seguro means I feel more confident / more sure of myself.

Me is the reflexive pronoun that goes with sentirse.

The full verb is:

• sentirse = to feel

And it changes with the subject:

• yo me siento
• tú te sientes
• él/ella se siente
• nosotros nos sentimos

So me matches yo. You cannot normally leave it out here.

Yes — it depends on who is speaking.

Seguro/segura is an adjective, and it usually agrees with the speaker’s gender:

• A male speaker would normally say: Ahora me siento más seguro...
• A female speaker would normally say: Ahora me siento más segura...

If you are learning to produce the sentence yourself, choose the form that matches the speaker.

Spanish often uses an adjective after sentirse:

• sentirse seguro = to feel confident / secure
• sentirse feliz = to feel happy
• sentirse nervioso = to feel nervous

So más seguro literally means more sure/secure, but in natural English it often comes out as more confident.

Because Spanish does not always need to say that explicitly.
Me siento más seguro already strongly suggests I feel more confident / more sure of myself.

If you want to make it explicit, you could say:

• Ahora me siento más seguro de mí mismo.

For a female speaker:

• Ahora me siento más segura de mí misma.

But the shorter sentence is completely natural.

Because Spanish often uses the simple present where English might use either I speak or I am speaking, depending on context.

• cuando hablo en clase = when I speak in class

This is a general, repeated situation, so the simple present is the most natural choice.

Cuando estoy hablando en clase would sound more like when I am in the middle of speaking in class, focusing on the ongoing action at that moment, which is less likely here.

Because this sentence refers to a habitual, real situation.

• cuando hablo en clase = when I speak in class / whenever I speak in class

With cuando, Spanish uses:

• indicative for real, habitual, or past situations
• subjunctive for future or not-yet-real situations

So:

• Ahora me siento más seguro cuando hablo en clase.
  = a real, repeated situation

But compare:

• Me sentiré más seguro cuando hable en clase.
  = I will feel more confident when I speak in class

That second one uses the subjunctive because the action is still in the future.

Because en clase is a very common expression meaning:

• in class
• during class
• in the classroom context

It often refers to the activity or setting in a general way.

• Hablo en clase = I speak in class

If you say en la clase, it usually sounds more specific, like a particular class:

• en la clase de matemáticas = in maths class
• en la clase de ayer = in yesterday’s class

So en clase is the most natural general phrasing here.

Yes. Ahora is flexible.

These are all possible, though with slightly different emphasis:

• Ahora me siento más seguro cuando hablo en clase.
• Me siento más seguro ahora cuando hablo en clase.
• Me siento más seguro cuando hablo en clase ahora.
  (less natural in this context)

Putting ahora at the beginning is very common because it clearly sets up the contrast: now I feel more confident.

Yes, you might hear that, but me siento más seguro is usually better if you want to express how you feel emotionally or mentally.

• me siento más seguro = I feel more confident
• estoy más seguro = I am more sure / more certain

Estar seguro often suggests certainty or confidence too, but sentirse seguro is especially natural when talking about personal feelings and self-confidence.

For this meaning, seguro is the safer and more natural choice.

• seguro can mean sure, confident, or secure
• confiado can sometimes mean self-confident, but it can also mean overconfident, trusting, or even careless because you trust too much

So me siento más seguro is the most neutral and natural way to say I feel more confident.

Yes. It is a very common and natural expression.

• hablar en clase = to speak in class
• participar en clase = to participate in class

If you want to stress taking part more actively, participar en clase is also useful. But for simply speaking in class, hablar en clase is exactly right.

You would change both the reflexive pronoun and the verb form:

• Ahora me siento más seguro cuando hablo en clase.
  I now feel more confident when I speak in class.

• Ahora te sientes más seguro cuando hablas en clase.
  You now feel more confident when you speak in class.

• Ahora se siente más seguro cuando habla en clase.
  He/She now feels more confident when speaking in class.

• Ahora nos sentimos más seguros cuando hablamos en clase.
  We now feel more confident when we speak in class.

Notice that the adjective also changes for number and sometimes gender:

• seguro / segura
• seguros / seguras