I have Eustachian tubes that don't function as well as they could, I can pop my ears when going up hills well enough but am curious if I use an aeroplane if ill end up flying in agony?

Yes travelling by air would be a problem for you, but many people with this affliction take a decongestant or nasal spray prior to take off or descent. This acts to allow the membranes in nose and throat to shrink allowing ears to equalize.

I saw a specialist who said I should be right because I can pop my ears but he left a little uncertainty. So are you medically trained at all? Do you think it would be a case of JUST pain but not popped eardrums or anything?

Popping your ear drums in flight will not give you relief for the whole flight and until you land. No I can't say if it would be a case of just pain or popped eardrums. You were at a specialist and he did not give you a satisfactory determination as you say, and no, I am not a doctor, but a certified personal support worker and registered nurses asst.

You can also chew gum during takeoff and landing.

I have bad tubes also and my ears are filled more often than not. I've never had any pain any time I've flown.