watasi ha mettani okoranai ga, daizina yakusoku wo nando mo wasurerareru to komaru.

は marks 私 as the topic: as for me.

In this sentence, 私は sets up the speaker as the person being described across the whole sentence:

• 私は めったに怒らない
• 私は ... 困る

Japanese often states the topic once and then leaves it understood for the rest of the sentence.

Because めったに usually goes with a negative form and means rarely / hardly ever.

So:

• めったに怒らない = hardly ever get angry
• めったに行かない = hardly ever go

Using めったに with a positive verb is generally unnatural in standard Japanese.

怒らない is the plain non-past negative form of 怒る.

Here, it does not mean only will not get angry in the future. With めったに, it expresses a habitual fact:

• I do not usually get angry
• I’m not someone who gets angry often

So it is more about the speaker’s general tendency than one specific moment.

Yes, here が works like but or however.

It connects two ideas:

• I rarely get angry
• but there is an exception or limit

This が is slightly more formal or written-sounding than けど. In casual speech, many people might say けど instead.

Because 大事 is a na-adjective.

Na-adjectives use な before a noun:

• 大事な約束 = an important promise
• 静かな部屋 = a quiet room

So the な is the normal way to connect 大事 to 約束.

何度も means many times, repeatedly, or over and over.

The も adds the sense of even that many times / as many as that and is part of a very common expression:

• 何度も言う = say it many times
• 何度も見る = watch repeatedly

So you can think of 何度も as a set phrase meaning repeatedly.

Here it is best understood as the passive, more specifically the adversative/passive of being affected.

So the idea is not someone can forget. It is more like:

• someone forgets important promises, and I am affected by that

This passive often carries a nuance that the speaker is inconvenienced, annoyed, or negatively affected.

That fits well with 困る later in the sentence.

This is a very common learner question.

In a basic direct passive, you often get something like:

• 約束が忘れられる = the promise is forgotten

But this sentence is an adversative passive, where the speaker is the affected person:

• 私は（誰かに）約束を忘れられる

Here:

• 私は = the person affected
• 約束を = the thing that gets forgotten
• the doer is omitted

So を stays on 約束 because the sentence is built around I am adversely affected by someone forgetting the promise, not around the promise itself as the grammatical subject.

The doer is omitted because Japanese often leaves it out when it is understood from context.

The full idea could be something like:

• 私は（相手に）大事な約束を何度も忘れられると困る

So the person doing the forgetting is usually:

• the other person
• someone the speaker is dealing with
• whoever is relevant in context

Japanese very often omits subjects and agents when they are obvious or not important to state explicitly.

〜と困る is a common pattern meaning:

• it causes a problem if ...
• I would be troubled if ...
• that would be a problem

The と is the conditional if / when.

So 何度も忘れられると困る means that if that kind of thing happens, the speaker is troubled or bothered.

This is often softer than saying something very directly like それは許せない or repeating 怒る. It expresses inconvenience or distress in a natural Japanese way.

Japanese often uses the non-past form for:

• general truths
• habitual situations
• conditional statements

So 忘れられると困る does not mean only if someone will forget at one future point. It means more generally:

• if someone keeps doing that
• when that happens
• if that kind of situation occurs

Because the sentence is talking about a general condition, the non-past form is the natural choice.