The really important thing you have to keep in mind is that sometimes we get so focused in on one goal that we never stop to ask ourselves if the goal is worthwhile. This happens all the time in marriages. One or the other feels the marriage falling apart and they decide they have to save it at all costs. They get so focused on that outcome that they never really stop to ask themselves if they should save it.
It might be difficult to hear, and many religions don't say it, but not all marriages should be saved. Sometimes there are simply too many problems to overcome. This is especially true if one partner has a problem. Just look at the recent spate of celebrity divorces due to infidelity.
If a partner is unfaithful once and truly and sincerely regrets it, the marriage may be saved (it won't be easy, but it is possible) but if it's a situation where one partner has cheated pretty much from day one of the marriage with multiple partners than that is indicative of a very severe mental problem and it's going to take a long, long time for that to be overcome, and it can only be overcome if that person really wants to change.
In this scenario ending the pain and suffering of the non-philandering spouse is the most important consideration. The marriage probably shouldn't be saved at all, if you're in this situation it's not going to be easy to save your marriage alone.
Another example: I have a friend who is a great wife and mother. Her husband is an alcoholic. He refuses to admit he has a problem or get any help. My friend wants her marriage to work and she wants to stay together, but unless her husband is willing to change even if she does stay it's going to be a very bad marriage and not a good example to her kids.
You don't want your children growing up seeing one parent being disrespectful and abusive and the other parent being a doormat and allowing them self to be treated that way. It can really teach the kids very bad ideas of what a relationship should be like. It's sometimes better to move on and hopefully find someone who can be a good parent. That will show the kids the right way to treat a partner.
If you really want to save your marriage alone, you need to think it through carefully. If your partner doesn't care about you or the relationship enough to try to save it, what are you really trying to save? A good relationship will never be perfect but it should be fairly well balanced where each partner is giving as much as they are getting.