You and I are in a similar place. I can do 150 km flat rides with no problem, but 1000 metres of elevation increase wears me out.
So the longterm solution is to practice by doing more. I've set a climbing goal in strava of 50 km vertical for the calendar year 2017. As of Jan 15 I have done 3,435 metres, which is well on track, but its summer here.
Technique helps - learning to gain momentum from any downhill or flat bit and then carry that momentum as far up the next slope as possible. Don't coast down grades.
Pacing is a big thing too - working your body to a point that is not your maximum, so you can maintain a speed up the whole climb. No point going mad for the first quarter and then needing a rest stop.
Equipment - climbing is hard work. I have an unfashionable road bike with a triple front, and a low gear that's below 1:1 (ie 26 teeth on the front and 28 on the rear cassette largest cog) Roadies look down on triples, but this is about YOUR performance, not theirs. Many MTBs had triple chainrings too, but 1x have become more popular lately.
In short, you might benefit from adding bigger gears, so that you can increase your cadence while climbing. Mashing is not as efficient as spinning up a steep grade. This might mean changing your cassette for a larger one, or having a choice of rear wheels available.
Weight - If you're lugging 2 bottles of water up, do you need them? Its hard to justify carrying stuff that you might need, but some things like pumps are non negotiable. Are you carrying too much food?
On the same vein - your bike's weight. I changed from a 20 KG steel MTB to a 12 KG aluminium road bike and my segment times all dropped by 10% immediately.
This image shows how much of your energy goes into different things as the slope increases. This one is aimed at MTB riders, road is worse.
Timing - riding in the heat takes energy out of you. Consider riding when its cooler, like earlier morning rather than noon or afternoon. Also, looking at the local weather forecast for a nice tailwind could help your climb.
Training - Doing specific training to target climbing, like hill intervals where you go hard-out up a grade for 30 seconds and then 2 minutes of relaxed climbing. Repeat 6-10 times.
Motivation - go out riding with someone who is better than you. Track your progress on climb segments in Strava, to see your progress. Here's an 800 metre segment at the top of a climb, where I got a PR today. You can clearly see a decrease in my times over the last 20 months. A bit more work and I might get to half the initial time, which would be awesome!
Other than that - practice. You can't improve without doing.