I think "first love" can be broken down into so many different things.
For me, my first boyfriend was just a boy I shared my lunchables with and we shoved each other a lot on the playground and held hands and said "this is my boyfriend".
The first "love" was my ex husband, who I was with for 6 years, but I don't think I really loved him, at least not in the way I love my current hubby. I thought I did, but looking back, I never got that sick to my stomach butterflies, fiery passion, even a second away from you is agony type thing. I have that now, so comparing the two, I consider my current partner my "first love", even if he wasn't my first boyfriend.
As for your loss, I am so sorry to hear that! I hope this wasn't recent, and if so I hope you find healing and discover so much more love along the way!