To get a good lap on a track like Spa, which is quite long, you need a lot of laps under your belt. Spa is a track of the old guard, there are long straights followed by medium difficulty series of corners then again long straights. It's Monza with more components and more woods
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Sector one and three are where most drivers find difficulty since there are high stress points, ie. T1-hairpin that sets up for Eau Rouge and the uphill climb. Lose time here and you're easily ruining your lap. Sector three same thing with the chicane, brake too late early and a bad exit and a great lap is lost. Focus on exit speed, carrying too much speed into a corner too deep will ruin that. Get a setup/car that you feel puts the power down with little drama at the rear end. Mid engined cars are fast but they drive differently and can be more difficult to get a hold of.
It takes a minimum of 50+ laps to get acquainted with the track, then by car you can add more laps depending on its driveability out of the box. Give yourself time, it's a bad idea to take top time setting setups, plug them in and expect greatness. Your driving style might be different, the setup might change the attitude of the car to suit the original driver. There are more factors that can be listed.
In summary give yourself time, you will see improvement.