I will keep it in mind that you were by my side when I was in my worst time of my life.
在我人生最低谷的时候有你陪着我,我会铭记在心
附：“铭记在心”翻译为keep in mind 或bear in mind.而不是"keep in heart"英语不这样讲.
it为形式宾语,指代“在我人生最低谷的时候有你陪着我”这件事.
“人生最低谷”可以翻译为in my worst time of my life或I was at my lowest condition of my life./I was at the darkest time of my life.
"by my side"即在.身边,或者用you keep accompany me /you were with me都可以表示在.身边.