Basically, you are attached to this person. You may be deluded into thinking otherwise, but so long as you have feelings and ideas about this person - that in your mind seperate them out from everyone else as being significant and special to you, then you are attached to them.
Now, there is nothing wrong per se about being attached and in love with someone, but this love you feel is suffering - it may not be apparent right now, but as soon as the conditions for your love of this person cease to be met, you will suffer, as soon as you are parted from this person you will suffer and so on. So long as you are aware of this then go head, be in love, but try to see it for what it is because being deluded about things will make it worse for the both of you when it inevitably encounters change.
The best advice I can give is to be kind and understanding towards that person, nothing is going to make this last forever - you should not think of it in these terms. You say you want to spend the rest of your lifetimes with them... I think you're mad
you'll be sick of them before the end of this life! Why would want that to continue in the next life?
Take this feeling for what it really is, and while it lasts be good to that person, do not see them as something belonging to you and don't expect anything from them, and don't expect things to last forever - you should be ok if you understand that.
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring."