I understand that your kids are very important to you and it's difficult to see the mother of your children involved with someone else. The first step is to have open and honest communication with the mother of your children. You need to express your feelings calmly and respectfully, explaining that you are uncomfortable with her seeing someone else. It's important to listen to her perspective as well, as there may be underlying reasons why she has started seeing other people.

It's also essential to focus on the children and their well-being. Remember that they are your priority, and their happiness should come first. If the new relationship is affecting your children negatively, you can express your concerns to the mother and suggest finding a way to co-parent and make decisions that are in their best interest.

Additionally, take some time for self-reflection and personal growth. Focus on developing your own interests, hobbies, and friendships. This will not only distract you from the situation but also help you become a happier and more fulfilled person overall.

Finally, consider seeking professional help, such as couples therapy or individual counseling. A therapist can guide you through the process of navigating a difficult situation like this and provide you with tools to improve communication and co-parenting.

Remember, it's essential to approach the situation with understanding, compassion, and a focus on what's best for your children.