Ride more! You can train in a gym but it is more fun on a bike.
You shouldn’t be too concerned about the gear you are using as long as your cadence is comfortable and you are getting up things. I’m a granny ring climber and it doesn’t slow me down. I just spin my legs while others prefer to push a big gear*
If you are getting saddle sore then it might suggest your saddle is not at the right height and your hips are rocking lots. Google saddle height for pics as it will help more than a written explanation.
*As above short sharp steep stuff is best attacked in a higher gear where as longer stuff is best tackled in a lower gear.
Look for the smooth line and grip to improve efficiency – no mud or loose stones.