Here's a few things that worked for me:

As others have said, just keep working at it. It will take time, but you will improve.

If you have low gears, don't be afraid to use them. Just don't go so low that your cadence is greater than 100.

At this stage don't worry about setting any speed goals for yourself. Set your goal to make it to the top of the hill without stopping. Go as slow as it takes to make this happen.

Last, and the most important, a good mental attitude is critical. If you think that a hill is impossible to climb, you will never make it to the top.