Practice, do a lot of practice races with the PP range that you usually use, pick a track where you are often losing, set a 3 or 5 lap race, run it yourself several times. Use the best car that you have the tune for.
Make a note of the best lap on each race, try to improve your time. This only good for solo run ( when you are leading ), if you want to polish your race craft ( braking bumper to bumper, side by side or overtaking or defending position ), you need to join race clubs or leagues which are on your skill level.
This is a good place to start, they are nice to chat, clean and courteous drivers :
https://www.gtplanet.net/forum/threads/crap-casual-racing-group.229656/
Another alternative is to do offline race against AI, use the super licence races, the ones with tire wear ( mile challenge and minutes races ), try to win by being clean, no contact and reduce power if necessary to make it harder, this will help you when being stuck in the middle of the pack or overtaking other racers. Picking lines and know when to back off, or when to put pressure
Tunes don't make winners, drivers do
You can have the best tune in the world, and if you got sloppy or falter under pressure on a tight packed racing, it's over
If you want to know if a tune is good, enable tire wear on very fast, do 3 or 5 laps race, see how the car perform on last lap. A good well rounded tune will be still forgiving even with reduced grip from worn out tire, of course this only applies if you race with tire wear on.