1. In order to have the same meaning how could I rephrase the following sentence?

I have beeen working on the project for more than a month.

It takes (or took ?) more than a moth since a I started the project.

“It is more than a month since I started the project.”
“It has been more than a month since I started the project.”

2. It wasn’t easy for me to make such a decision.

It is correct to rephrase in the following way?

This was not easy thing for me to make such a decision.

3. As Peter won the match he was very happy.

Peter was very happy when he won the match.

4.If you behave like that, people will avoid you.

The more you behave like that more you will be avoided by people.

5. If you keep eating so many sweets you will get fat.

The more sweets you eat more fat you will get.

6. It is necessary for us to leave at once, or we’ll miss the train.

We should (or have to) leave at once, if we don’t want to miss the train.

6.You are asking these questions because you didn’t listen to me.

If you listened to me you are not asking these questions.

Making this decision was not easy for me.

People will avoid you if you behave like that.

We’ll miss the train if we don’t leave at once.

As you didn’t listen to me you have to ask these questions now.

Are your answers better alternatives or my phrases are totally wrong?

In most instances your answer slightly change the meaning of the original.

Give me more explain for phrase no 6.
What is gramatically correct “have to” or “should”

‘Have to’ - it is necessary
‘should’ - we ought to

So to maintain the meaning of the original sentence, ‘have to’ is needed.

‘Need to’ would also be correct.