ha vs. wa?

Hi everyone, I'm really confused as to why (in hiragana) the 'ha' character is used instead of 'wa' in some words e.g. konnichiwa, konbanwa, dewa arimasen etc.
Any help with this would be much appreciated!
Arigatou.

'ha' is used as a particle and read 'wa'.

'kon-nichi-ha' thelast 'ha' is a particle.

kon-nichi-ha ikaga desuka? some words like this is ommitted, meaning 'how is it going today?'. You know kon-nichi is 'today'.

kon-ban is 'tonight'.

You may want to say 'kyo-ha-samui desu', 'today is cold, which means you feel cold today' to the question like 'how is today going?'.

A particle {wa} is written as "ha."
A particle {e} is written as "he."
A particle {o} is written as "wo."
These are, so to say, remains of historical spellings.

It is considered that in the middle of Heian period (794 - 1185), people began to pronounce "ha" like {wa} in some cases.
The spelling rules of Japanese language established in 1946 replaced such "ha" by "wa" except for the particle {wa}, which remained unchanged.
Similar things can be said about "he" and "wo."

The particle {wa} works as a topic marker.
Greetings "konnichiwa" and "konbanwa" are short forms. Originally these phrases were used in sentences like "Konnichiwa yoi otenki desu." ["Today it is fine."] and "Konbanwa yoi ban desu." ["Tonight it is a good night."]; as time went by, people began to use the first phrase as a greeting.
"Konnichi" means today and is used now in a stiff or an archaic style.
"Konban" means tonight and now may be used also in daily conversations, but I feel "kon'ya" more casual.

Very good explanation above, and I'd add..

It's not too much of a big deal with a little practice. Just acknowledge it, embrace the idea, and soon enough you won't think twice about it.

I think to the foreigners, it could probably be because a word ha(は) is not written as ha should be in Romaji, people get confused with a word ha(は) to wa(わ) or wa(わ) to ha (は).
The words ha(は) and wa(わ) would make a difference if you are writing it in Japanese.
It's simpler to understand if the words are differentiated in such words as in hiragana,
wa(わ)tashiha(は)=わたしは
haa=はあ
waa=わあ