I know this isn't the advice you are wanting.
Does your partner agree with you taking a mistress? If not - or if you're doing it secretly - she really needs to know that you not having your needs met is making you feel like this. Having said that, she needs to realise that the relationship is at crisis point.
Get yourselves into marriage counselling pronto. Or end the relationship. If you're good marriage material, you shouldn't struggle to find a new partner.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.