As infatuation means attraction for someone for short-time. So is it correct to say "he has always been my infatuation"? Can I use "always" in that sentence. Or is it correct "I have always been infatuated with him." Again here can I use "always"?
I prefer the MW definition of infatuate:
1 : to cause to be foolish : deprive of sound judgment
2 : to inspire with a foolish or extravagant love or admiration
The notable thing about an infatuation vs "real" affection is not the duration but the foolishness of it. Short duration is just one possible sign of this.
In common spoken English, it's more common to say "He/she is infatuated with him/her" rather than name someone as the infatuation, it's just not done and gives a very objective tone.
So you'd probably say "I've always been infatuated with him" however it's important to note that if you've always been infatuated, then it's not short term and it's not infatuation.