What does 看得开 and 看不开 mean?
Just so you have another interpretation. As far as I read the sentence, 看得开 means "letting go" and 看不开 means "clinging on". Another hard point in this sentence is the structure "... 是好 ...也...". The original structure should be "...也好...也罢" which can be construed as "either... or..." or "for better for worse". Here 好 does not mean good. It just mean "if it is so". So the first sentence means that "either you let it go, or cling on it, you will get through". So what the sentence actually means is this:

许多事情，看得开是好；看不开，终归也要熬过去。别以为看不开就不会过去
A lot of things in this world will of course fade away if you let it go; yet even if you simply cling on it, you will still get through. Don't you ever think that it will never end as long as you cling on.

My translation is very crude, mainly because I can't convey the same tone as it is in Chinese. This sentence have a really Taoist tone that for better for worse, things will run on it's own course. It doesn't matter how you think of it. The only difference is that if you cling on it, you need to endure more (熬).