How do I stop overthinking in a relationship and maintain a long-lasting relationship?

You get busy with your major transformative purpose in life (it just makes you a better more interesting person), and then when you think about your relationship, you ask yourself “what could I do today to put a smile on his face.” Then you give up your expectations and accept whatever comes your way as a gift.

If, having done that, you find yourself at play with your partner, communicating what's so, happy to be coming home to this person, and he feels the same, you have decent grounds for a long-lasting relationship.

You also want to make sure you are aligned as to your visions of the future. Ask, have conversations about kids and religion and money. Make sure you know your deal-breakers, and his, and that you are both okay with living inside each other's visions.