Firstly - thank you, everyone. I am so touched by your responses, I have a lot of guilt and sadness about my feelings and don't really have anyone to talk to about it, besides my partner who can only try his best to understand.
TheMumsRush - thank you, not feeling very lovely at times! Obviously my partner and his children have been through a lot, and sometimes it feels like I'm just making things harder. How pitiful I sound 
Petal02 - thank you, the fact that the children are so young and that there are 3 of them very close in age is extremely daunting. I never even had much to do with children before I met my partner (since being one myself, that is...)
Eliza - thank you, I really really hope that I do. Completely forgot our ages - I'm 25, partner is 39. You are right about asking whether I can sacrifice my own hopes and dreams - lots of things that I took for granted, like living with a partner for a while before becoming a family, having a choice in how big a family we wanted, being free to move house if we wanted or live abroad... as much as I want a future with this man, it's hard to picture as it will be so far removed from anything I ever anticipated for myself.
daftgeranium - thanks, so glad others feel the same way. I have told my partner how I feel about us having missed a stage in our relationship, and feeling sad about that. He tried to say that we would have time alone together in the evenings, but I pointed out that the children won't be going to bed at 7.30pm forever! And then he agreed that yes, the expectation would be that evenings would be spent together as a family. His parents and sister are supportive, and they provide the childcare every other weekend, but it is a big ask of them to look after 3 small children so often, and considering how long we will likely be long distance for, the frequency might have to be cut down.
Lookingforthebrightside - thank you, sounds like we found ourselves in a similar situation. I'm sorry to hear how it ended for you, but quite a testimony to you and the children that you have such a good relationship. I'm studying at university at the moment, and will be for another 2 and a half years - my aim is to work afterwards, and my partner is agreed that childcare would largely be down to him, in terms of the day to day school runs and so on. He has no expectations on me in terms of looking after the children, apart from to be caring and loving and for us to be a happy family. If only it were that easy!
riverboat - thanks, you hit the nail on the head with the vicious circle comment. That is exactly how it feels, and when my partner and I discuss the situation, we just go round and round. Ideally, I would be nearer, and would be able to pop in for a few hours / the odd evening, to get a better feel for the situation. As it is, it's hard to know how we will ever feel secure that it can work. Sometimes I think I should walk away, when I feel sad about the long distance, scared about potential future step children, when my partner is sad because of me - but I always come back to the fact that I love him more than I have ever loved anyone, and that if it were just the two of us I genuinely believe we would have a very happy relationship. Hard to walk away from that. 
GiveItYourBestFucker - thank you, so you've been in the holiday situation too! I have never felt so isolated, frustrated, bored, scared... my poor partner, part of me wishes I could have kept a bottle on my feelings until we got home but I was so devastated to suddenly feel like I was out of my depth. I'm sure a lot of mums do wish for bedtime, I know my partner is glad to have time to himself.
moreyear - thanks, that is exactly what I miss / think I will miss - memories of that time before you had children. We are going on holiday together in a couple of weeks - it will be the longest time we have ever had alone together, so might be a bit strange! We try to do nice things and build some nice experiences together, but to be honest a lot of what I miss already is just the day to day things. Coming home to your other half, having dinner together (uninterrupted dinner), lie ins at the weekend... so much so that lately, on our weekends together, I have preferred just being at home to going out and doing anything - just that little bit of domesticity is what I miss! Not so good for building memories together though...
I'd like to say again that I really am grateful for all of the comments. I've had a lot of sleepless nights over this, and realise that there no real answers, but it has helped immensely to hear your thoughts.