My lover/Master has always been poly, and he warned me from the start that, even if he stays in love with me, he will, eventually, need a little "strange." I, too, am insecure-he has much more experience, vanilla and BDSM than I do, and I worry that I will not be good enough or even enough for him. So far, he has been very supportive/complimentary towards me-he says I do things no-one else has-but I still worry that he will want/need someone better (or thinner, or younger, or braver, or just more than I am.)
We have agreed that he will warn me before he finds his "strange," and I have said that I would prefer if WE found the strange together, I think because if I see him with her, I will know by his behavior how he feels about her. I also am, what's the term, bi-curious? At my age (42, and proud of it) I hesitate to try and find a female/female relationship because I'm a little too old to fumble like a teenager. If the two of us took a third together, he could help me with what to do with her, and I would look like less of an idiot. If we found another female sub, he could teach me about topping, so I could do a better job of topping him when he wants/needs that (he is a switch-top, bottom and vanilla.)
The thing is, I know, as in have met, other women who want him. I know they exist, are not just phantoms of my fear. I just remind myself that he is with me now, not them, and try to rein in that green-eyed monster. I know that if I am too possessive, I will just drive him off. Dog, as you may have guessed, is a nickname, but it reminds me that he is not a lap dog and would resent a leash. Okay, this may be hokey, but I try to remember the old 38 Special son, Hold on Loosely, and live by it with him.
I don't know if these thoughts help any, but they should at least give food for thought.
(BTW, Oz, I get the 1 to 10 years comparison, especially since I have three lovers and one of my best friends all in one person.)