I am going to dotake on a new challenge about my job inin my career this April. I am going to take on a new challenge in my career this April.

We don't really say "DO a challenge" but rather "undertake a challenge" "take on a challenge" Rather than "about my job" (this sounds like "to do with my job") maybe "in my career" We don't say "IN this April" but just "this April"

I getThis makes me a bit nervous because I like simple days and I don't like change. This makes me a bit nervous because I like simple days and I don't like change.

"I get a bit nervous" is grammatically right, but it would sound more natural to clarify that the challenge makes you nervous. Maybe "easy" instead of "simple"?

but, I think it will be an opportunity I will beto get better. but, I think it will be an opportunity to get better.

"To get" because it is an opportunity to help you travel/move towards later BEING better.

So, I decided Ito accepted this I will make ito get better. So, I accepted this to get better. So, I decided to accept this to get better. So, I accepted this to get better.

Either of the sentences above would work. Saying "to get better" ties together the sentence; otherwise it sounds like two separate sentences.