The key to big climbs is pacing. You can't go too hard at the beginning.
You can increase your aerobic threshold by training at or a little below threshold.

The other thing about big climbs is that you can't stop pedalling. Getting used to that will help.

And last is the mental aspect. If you fear or dislike the climb and spend your whole climb staring at the road in front of your wheel, it's going to be miserable and you will be more likely to quit. Embrace the climb and enjoy the view on the way up.