I think it's pretty normal to mourn the end of breastfeeding 'but', I think the thought of it is far worse than the actual process of stopping.
I was really into breastfeeding and I always thought that both of mine would be going to school with a flask of breastmilk but both of them self weaned and it felt okay.
I think it's a really good observation to say that you're worried about losing your identity and purpose. It's like when you're pregnant and you're part of a special club then you have your baby and you're straight into another special club. I think breastfeeding is the same, you're a breastfeeding Mum and you're part of a special little club in a very special bubble... what will happen when that's over? Well, you'll move on and find pride and satisfaction in the next thing. Instead of tugging at your top she may need to snuggle into you with her bedtime milk or she may have become a little comedienne and have you in stitches. Something great will happen and you'll find it easier to let go than you imagine.
You really WILL find it easier than you're expecting. I still get a pang when I pass a breastfeeding room, part of me still want to stick my head around the door and say "I used to do that!!!" .... but I don't
It's lovely that you're going to feed for as long as your little one wants to and nobody will ever be able to take that away from you. In ten years you may have decided to take up latin dancing or karate or maybe you'll have flying lessons and everyone will know you as Jen the Pilot. Who knows what your identity and purpose will be then?

Don't worry, let it all happen and see where you end up.