"Why am I losing sexual interest in my partner?"
Losing sexual interest/attraction towards a partner is not uncommon or abnormal. The loss of sexual interest (also seen as low sexual desire/libido) is influenced by a variety of variables that can be psychological, relational or biological. The most important thing to know is that loss of sexual desire or attraction is often a symptom — not the root problem.
How to have good sex.
Sex should be good, and what society deems 'good' is very misconstrued.