OK you peeked my interest and I noticed you listed your bike in your profile (well done).
A Trek Pilot 1.2
The web claims 52/42/30 front, & 12-25 Cassette on the rear.
With a small 30 tooth Front & big 25 tooth rear, that's pretty good for climbing.
alcjphil Pointed out my error above.....
small 30 tooth Front & big 25 tooth rear, the easiest gear, could use some help.
I myself use 34 front-34 rear & 34-36 on my bikes.
Unless you are prepared to learn a lot about how the tooth limitations/specifications work on derailleurs, a trip to the bike store for options might be in order.
It's generally not just a case of finding a larger rear cassette, other spec's need to be taken into consideration.
------------------------------------------ -may also be
I suspect it's low RPM and/or technique
^ causing you some troubles.
Barry