I think one way to tell if what you feel is love is… how would you feel if the person left you for another?
Could you be happy for the person, if their choice was a free one?
Could you let the person go without trying to sabotage the new relationship, extort the person back, or punish in other ways big and small, overt and covert?
Could you still love the person, knowing that the feeling was not reciprocated, or at least not with the same depth or passion or on the same schedule / timetable?
Could you put the other person’s happiness above your own in this case? In all cases?
Can you give the other person whatever time is needed to reach certain realizations and conclusions without demanding answers, conformance, resolution and performance on your timetable?
I think if you can answer “yes” to those questions, then what you feel is a real and abiding love. Good luck on never having to actually answer those questions in the real world. I’m learning those questions and answers because I have been dealing with them in several ways (except that she hasn’t left for “a person”, and she may be ambivalent about leaving in any case), so in my case I can say “I know what love is.”