to begin with为惯用法。《麦克米伦词典》很清楚地解释了其二种不同用法。请体会例句。

1  SPOKEN used for introducing the first thing in a list of things you are going to say

‘Why were you annoyed?’ ‘Well, to begin with, I missed my train.’

2. before or during the first part of an activity or process

    How did you get involved to begin with?

第一种用法中，to begin with为评注性状语。第二种用法中，to begin with为时间状语。

网友句子中的用法属于第二种。

你的句子难点在 to begin with，其余的结构比较简单：

你的句子中的 to begin with 为插入语，其实也可以将它移到句首：

It was great to begin with but now it's difficult.

=To begin with, it was great, but now it's difficult.

又如下面各句中的to begin with 也是插入语（注意with后面是没有宾语的）：

I liked it inAmerica to begin with. 我起初是喜欢美国的这一点的。

To begin with, I do not like its colour. 首先，我不喜欢它的颜色。

I found it tiring to begin with but I soon got used to it. 刚开始时我觉得很累， 不过很快就适应了。

但是，下面句子中的to begin with 就不是插入语了（注意with后面有宾语）：

I have to begin with an apology. 我得首先表示歉意。

You do not need to begin with a formal greeting. 你无需以正式的问候语开头。

这是《柯林斯高阶英汉双解学习词典》第8版 begin 词条中的一个例句。在该例句前面的说明是：You use to beging with when you are talking about the first stage of a situation ,event, or process . 最初；刚开始

You use to beging with to introduce the first of several things that you want to say. 首先；第一 What do scientists you've spoken with think about that? --- Well, to begin with, they doubt it's going to work.“跟你聊过的科学家是怎么看的？”---“嗯，首先，他们怀疑这能否行得通。”（摘自《柯林斯高阶英汉双解学习词典》第8版 begin 词条）