Power to weight ratio. At 18st you're never going to be as fast up hill as the fit guys half your weight.
I'm around 16st and hill climbing is definitely my weakness when racing, although in a mixed group non competitive fun ride, I reckon I'm faster than most up hill.
The way I train hill climbing with slower riders is to go on ahead to the top, turn round, ride back down to the back marker, ride back to the top and repeat.
Can you get one of your faster guys to do this ?
You can turn it in to a game. He tries to "lap" you 4 times, you try to catch him before 3.
It needs everyone involved to be willing, otherwise it just looks like the fast guy is showing off or trying to wind up the slow one, but at least it means no one gets left at the back on their own.

Old – might take you up on that.
I've got to sit down with the map and fathom out some rides as I'll be going longer distances in 2-3hr sessions than on the mtn bike.
She's an old bike, 3 rings at the front and a one-piece stem, which I hope is non-fatigued or the first pot hole is going to see me over the bars!

Sarratt, Bovingdon/Botely, St Leonards, Wiggington, Tring, Ivinghoe Beacon, Studham….don't know the rest but you get the idea 😕

power to weight and setting off too quicky. While not as heavy as you I am heavier than the folk I ride with mainly – as soon as the slope goes up I drop behind them even tho I am faster on the flat ( where power counts more than power to weight)

Go at a pace you can sustain from the bottom – you will get to the top quicker than setting off too fast and blowing up

Bumhands beat meto it. Don't look too far ahead,find a gear that's comfortable and concentrate on keeping your breathing nice and steady.
Don't consider stopping.
I've got the same bike,but I'm 10 and a half stone. I love climbing on and offroad.

rockitman hats off to you for climbing anything on a 575 or any susser. That's why I ride a xc racer I think my head would explode carting that stuff uphill.

Height to weight ratio – I remember hearing that a good climber ratio is 2lbs for every inch of length.

My technique for long draggy climbs is… 1. don't look for the summit – it'll make you crack, and 2. when pedalling count the downstrokes in 3's (ie 1,2,3,1,2,3,…) so the "1" will alternate between each leg, but don't put too much stress on that stroke (hope that makes sense).

When it gets to a boring sections, I count my breathing (not the same as HR!).
It passes the time surprisingly well and allows you some idea of how your work rate is, just keep it steady and not increase it as that's a recipe for disaster if there's a long way to go.

It's definitely down to fitness, but with that sort of build it'll never be your strong point: