Hi Katka, welcome to seabreeze!
A few questions (so we can give you the right answers)...
What gear has the rental place been giving you? (Sail size, standard diameter or reduced diameter mast)?
Are you hooking into the harness lines yet? (The sails a lot lighter and easier to handle once you start doing this)
You can to a degree "lighten the load" with some gear choices, but once you're using the sail the right way, the only times you'll notice how light it is, is up hauling and carrying it up the beach.
The good news is that as you progress it gets easier and easier. Hooking the harness lines means next to no load on your arms as you sail along, water starting is ten times less exhausting than up hauling, gybing is 50 times less exhausting than falling in and having to uphaul/water start