Some people can climb hills and others struggle. I have been riding for 15 years and in some Groups I am the slow rider, others the slow rider up hills, and even with some- I am up amongst the leaders. What I do find though is those that can climb- can rarely descend.

Building up leg muscles and in particular the quads is a must. I have quads to show up any weight lifter but I still do not climb fast. What I do though is actually climb the hills. Unless I lose grip, I never walk a hill. Gets a bit annoying that those that do walk catch me up but that's the way it goes.

You have it wrong with the fast riders in that they are hardly breathing. They have learnt the trick and you haven't. They are breathing hard but only show it when other riders are not about. If you are breathing hard- Slow down until not only the legs get better- but the lungs do too. Or control the breathing when other riders pass you.

Incidentally- the hills do not get any easier-- They just take less time to climb