There’s a lot going on your post.
Re the grieving of the loss of the family that you feel you should have had? I think that is quite common. Certainly I went through that. I’ve now been separated/ divorced for 5+ years and theee are still some days when I look at my situation and feel sad. My life is not what I planned for. But for me, the feeling passes quite quickly. (I’ve had counselling post my separation).
Re your emotional load? Yes it is hard. Feeling that the buck stops with you now on all fronts is quite an overwhelming feeling. Financially, logistically etc. Running a home alone is hard work. As is doing the bulk of the parenting / school runs etc.
You don’t say how old your children are, but there is a huge difference between parenting a 4 year old and a 12 year old. Mine are teen/ pre teen and I can honestly say it has gotten so much easier. It does get easier as they get older (in my experience) - as they can also start helping themselves with chores (my eldest can now cut the lawn for example and does for pocket money).
So it won’t always be this hard.
In terms of expectations, please read some of the threads on here for people who have attempted to blend or are blending. It is not an easy situation. By any means. Blending is not a magic bullet, it requires a lot of compromise, strong communication skills and (honestly?) time to see if it is even the right scenario for both families (especially the children).
I’m not going to lie, your set up sounds ideal. But I can say that as I’m operating from a place of strength and I wouldn’t even think about blending / living with someone now until my children are grown up. I also (selfishly) have no need to share my home with someone else’s children.
What I can remember post separation (pre counselling) is feeling how you are feeling. I then met my ex and in hindsight was desperate to recreate a ‘family’. I think it was more for me (I can also align with the emotional overwhelmed/ loss of my family). What Can i advise? Give it time, I have adjusted to my divorce. I love time with just my children and I. I feel utterly complete when it’s just us (at home relaxing, going on holiday etc). It takes time to adjust, it’s important to grieve your loss. But this feeling (in my experience) does not last.
I think your set up sounds ideal. It sounds like the person you have met is stable himself. It’s lovely that he helps you the way he does. And he sounds very boundaried with your children - it’s not his place to do anything ‘father’ related with your children. That may come with time, it may not.
Personally I wouldn’t rush anything and I would work on yourself. Loss of family unit is huge, don’t rush to replace it with something new x