When I went on group rides with my friends, I hit a "wall" as well on a 1.5mile 10%+ incline. Needless to say, I had barely made it to the top. I even had to stop in the middle to catch my breath and made my group wait for me at the top. To combat this, I started training on my local steepest hill. Between 10~22% of hell over a 0.5mi incline. I had my 34t crank x 25t rear. I managed to get up the hill halfway, then once, then twice. then I changed my rear cassette to max at 28t. Now I try to hit the hill 4 or so times. On hard group rides, I tend to stay seated unless sprinting to the top for a segment. High cadence, lowest gears, and constant burning lactate in my legs. The more your legs are exposed to that lactate, the more resistant to lactate fatigue they will become!
While some people may bash you for using mountain bike gearing, I will not! Do what it takes to get to the top with your legs and your lungs intact! but at the same time, keep doing hills! We all have different bikes and fitness levels.