It is possible you may have a sex addiction or some deeper emotional issues that need to be dealt with. Some marriages and relationships survive affairs reasonably easily and others don't. Some partners even agree to allow affairs in their relationship. With this type of relationship they may stay together and share financial responsibilities and even children but both are having affairs and that is accepted. If you feel that you can never be happy tied down to one person and need to have affairs then perhaps you need someone that is open to having this type of open relationship where you have an agreement that you can both have affairs. This way you are both doing the same thing and no-one will get hurt. If you have had an affair and your partner has found out, the relationship isn't necessarily over. Many relationships have survived affairs and if you and your partner have a strong commitment then this affair may not have to be the end of your marriage.
It isn't all that uncommon these days to have an affair. Is it what your relationship needs?