Ex partner galavanting the world while I raise the children alone

And then a hero came along........
Calm down, its not goady at all. I'm making the point that OP has by far the better deal.

It is hard. 100%.
I suppose I just try my best to remember one day I will want them wrecking the house with toys everywhere or crawling into my bed, because one day they'll be grown and I will wish they were this little again. And when they are grown, I will be the parent they call, the parent they want to visit, hopefully the grandparent. Whereas he will be a twice a year relationship with no real understanding of love and to be loved. I know what I would prefer. But I also like to sulk about it too!

Agreed. Mine are young teens and I’m a sole parent too. I was worried for a while about how bitter and resentful I feel towards their absent father, but I decided that I was giving him too much power over me.

His actions (or lack of) are outwith my control and complaining about how unfair it is doesn’t change anything. I’m not saying I don’t think you should complain, I mean just to be mindful of how much time and energy you waste over thinking about hm.

I’ve had to consciously change my thoughts to other things including knowing that when they’re older, they’ll have a continuing close bond with me but I very much doubt they’ll have anything to do with him. It really is his loss because they’re brilliant kids.

You would rather go to some hotels for work than have your children with you?

One of DH's friends broke up with the mother of his 2 year old. We felt so sorry for him, he acted really heartbroken and mad her out to be crazy and unbearable to live with (but not too crazy to not trust her with his child).

But then about 2 months after the split, he was telling everyone the benefit of splitting up is he now has an amazing amount of time for himself and hobbies, he was training for a triathlon and was on Tinder already.

You just deal with the cards you were dealt with. It's shit.

I don't think he'll have any massive regrets either. Sure, he'll complain his kids don't call enough but he won't be heartbroken.

There is no fairness. You just remember those kids deserve all the love in the world and you do that for them.

He’ll forever have a hole burning into his soul
that he might not be aware of yet but it will rear its ugly head when he’s old and alone.

I honestly don’t know how you let go of the resentment, but just wanted to say you feeling that way is totally valid.
my ex works, socialises, buys new clothes every week, always in the pub and sees our daughter for a couple of hours and never overnight. He regularly tells me how he lives hand to mouth and has no money. He takes no responsibility for our daughter but acts like father of the year to others. I try very hard not to think about it because when I do it makes me so angry.

That sounds really really hard.
Does he pay child support? Do you have a close relationship that you could tell him you need a break sometimes and for him to take them for part of the holidays or the occasional weekend?