Why do i never feel like sex with my partner, its like i can never be bothered, i dont really get sexual feelings either yet if i have amphetamines i seem to be fine?

Stimulants: Artificial Psychostimulants induce release of (among other things) endorphins (which relieve stress, enhance pleasure, and improve mood). Which almost sounds too good to be true, right? Because it is. Keep in mind that the body always wants to remain in balance. Chronic use results in the brain counteracting those effects and many other problems. See a therapist - much better for you.