Infidelity, for most couples, is an unforgivable sin. However, change is the only constant thing in this world so you still have a fighting chance to change your spouse's mind if he or she is inclined to think along these lines.
You see, the problem with infidelity is that it is a breach of TRUST. And once trust is shattered, on what grounds then can you convince your spouse to take you back?
Give yourself time. Although your first reaction is probably to just get on your knees and ask your spouse to take you back, it is very important that you evaluate certain things for yourself first. Think about it this way - to be able to move ahead, you need to know where you are exactly right now. As such, ask yourself the following questions.
You really do not want to hurt your spouse a second time do you? As such, be true to yourself first and ask yourself if you really want your marriage to go on and why in the world should your spouse take you back.
Give your spouse time. Time cannot make one's infidelities disappear into thin air but it can make the pain bearable. As such, always remember to give your spouse the time and space he or she needs because your partner also needs to figure out what he or she wants from you (and life in general) from this point on. To push your spouse now into taking you back is counter productive. Sooner or later, your partner will see things in a softer light and be more receptive to listening to your explanations.
Step into your spouse's shoes. If YOU were the one cheated on, what would you 'demand' from your spouse to make you accept him/her back? You see, as the one who has strayed, it is easier for you to just beg for forgiveness and convince your spouse on the merits of taking you back. However, if you try and put yourself into the shoes of your spouse, you will, hopefully, see his or her concerns and so you can start planning ahead and start finding ways to address his/her issues.
Show your spouse the person he/she fell in love with the first place. As mentioned above, it is important to give your spouse the time and space he or she needs. However, while your partner is in this phase, gently show him/her reminders of the person he/she fell in love with in the first place. Did your spouse love your humorous side? Bring a little humor to your conversations every now and then.
Infidelity is one of the hardest things a couple has to face. However, if you are sincere in getting your spouse back and show him/her how great your marriage can be again, everything is worth trying isn't it?