For context I'm lesbian and I used to simp/nice guy for several ex gf which obviously resulted in me getting my heart broken many times.

Here is what I've learnt.

Rule number 1, being good to your partner is not a quality. It's the bare minimum.

There is a difference between treating your partner well and syncopathic behaviour like letting them indulge in their own bad behaviour. Learn the difference.

Don't just focus all your energy on your love interest/gf/bf/partner. Would you have found them attractive if all they did was obsess over you? Pursue your own passion, eventually people will find your passion interesting too. People like driven people, it's sexy.

As superficial as this may come across, don't be a slob. Take care of your appearance. You don't have to look like you just walked out of a magazine but learn to dress and groom yourself like you love yourself.

Don't expect someone to love you if you can't even love yourself. It's not narcissism, it's self respect.