I love Guinness, he's such a star!

what an angel with the littlies!
He's looking fab in his new saddle, as are you m'dear!
Just a couple of things to help you get sorted in it...
- need to move you seat forward a few inches, you're pushed to the back a wee bit. Probably due to the wide feeling it gives (especially on a coblet, I know how you feel, I have two wiiiiiiide beasts and now have hips of elastic but its taken some time!). By moving your seatbones forwards, you'll be more central and in the deepest part of the seat, so keeping the pelvis upright becomes easier.
- drop your stirrups a hole or two. This will let you get your legs under you and not so in front of you (also helped by shimmying that bottom forwards a bit, see above

) . Vicky's tip about 'no stirrups and toes down' will help too, it relaxes the upper thigh and calves and lets you weight down through the leg rather than you gripping up with any part of the leg and unbalancing yourself.
- opening the hips a bit more (allowing the legs to widen from the top - if that doesn't sound too rude!) will let your leg go 'round' him a bit better, letting you use your leg more effectively rather than trying to use your heels on his sides. Try to use your calf muscles on him not your heels (remember that one Vicky??? 8)). This will improve your impulsion and stop you gripping up and unbalancing yourself.
- hands, arms, upper body posture, all good..... BUT LOOK UP! you're doing a grand job, enjoy yourself and stop worrying!

have fun honey, hope some of that is helpful in some way,
Lucie xx