I understand how you are feeling. My oldest quit at 13 months suddenly which was shattering at the time. And I weaned my second at 18 months. It wasn't relly a crutch for us because they slept on their own, but both times I was very sad. Especially the second time, knowing it may be my last baby. But I too it felt it was time for us both.
I guess that the way I looked at it is that it was a very concrete way of moving on to the next stage of parenting. I tried to embrace it as having a "big girl" toddler. It is one of the first stages of separation to some degree. But I tried to focus on transition ot new things. For instance, instead of nursing my daughter showed more interest in cuddling and reading books which is a new level of fun to experience with her. Instead of sharing that gazeinto each other's eyes, we "talk" about a book and giggle, and there is a different sort of give and take. I also admit that I reallly like owning my body again.
I'd also add, that I don't think that the kids feel bad or miss it as much as we so, as long as it is gradual and you still have enough cuddle time. There may be the time he suprises you and asks ot nurse, but for the most part, sadly, I think they forget pretty quickly, which makes it easier on us.
So, I think it is normal to feel sad about what has passed, and as I write this, I feel a bit weepy thinking about it all. But don't feel guilt. You should be proud of doing something that is very hard for many women. You gave your kids the greatest start possible, and you have special memories of them tha tno one else will. Enjoy those, and welcome your new relationship as he grows up.