I so understand where you are coming from. I still have to say the L-word to my boyfriend. I overused it during my first relationship ever, as I was so 'in love' with the concept of love and I projected it on my first boyfriend back then. It was fun in its own way and certainly true to some extend, but I could never again be so carefree about it when this relationship ended, because I learned what the true meaning was by this experience.
During that period of time, the feeling of the 'I love you' changed, it became much more meaningful and honest, than it had been before and that's when I realized that I had thought to be in love before, but it was more a mixture of attraction and fondness and 'liking'. That's why I told my husband "I love you" for the first time a bit over a year after we officially got together. It was the earliest time I felt ready to say it and mean it with all that comes with it.
With Lin it is a strange thing, we have been 'in love' for years now, but because of all the things going on besides this knowledge, I am still hesitant to say it. This may be due to the thing you stated: There is so much responsibility in this little sentence. I thought it more than once, but it didn't feel right to voice it each time it came to mind. I am kind of curiously observing when my words will match my feelings for the first time
Therefore, as a fellow not-over-or-easily-user of that special word: Congratulations on reaching that level of comfort with Dude
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.