I have found that most people severely underestimate how difficult hills can be. The usual frame of reference is in a car or something, where the perceived effort going uphill does not vary much from the flats. Whereas when you're on the bike, the perceived effort going uphill is many, many times more than on the flats.

Fact is, hills are hard -- a lot harder than flats. It's normal to be "gassed" near the top and need to back off, especially if you are not doing any specific training to improve your hill climbing ability.

If you want to get better at hills, one thing you can try is intervals, where you go for X seconds as hard as you can, then recover for Y seconds; repeat. Start with X of 30 and Y of 90, then increase X and decrease Y over the course of several weeks/months. You *will* improve. But it takes time and some dedication to a program. Riding the same old course at the same old intensity all the time will maintain, but probably not improve your fitness.