Do you have any videos of you playing Pit? That would be the easiest way to help you out.
Anyways, Pit wants to get grabs. find ways to condition your opponent to get grabbed, such as using his amazing dash attack (fast, disjointed and does 11%), jabbing their shield (stick with the 3 hit jab opposed to the rapid jab, they can fall out of rapid and the 3 hit is really good ) and baiting with short hops and movement.
Your up smash is great at covering opponents landings, so try reading where they are going to land and let the up smash rip.
As Scruffy said, d throw followups will stop working as well around 70ish% (60 on floaties and 80 on fastfallers) so just throw them towards the ledge for stage control when d throw stops working.
Forward throw is a pretty reliable way to get kills at the ledge around 110% and so on, especially if you're playing d pit and they're fearing your side b. Up smash is a reliable kill option to cover landings. D-smash can also be a good way to kill at high percents, due to it being very safe on shield, relatively safe otherwise, and covering both sides.
as far as edge guarding goes, it's generally better to just wait at the ledge and harrass them with arrows and cover their get up option rather than chasing them off stage hastily. There will be situations it is better to do that, but have stage control in mind more often than not.
Nair is good out of shield option because of it's startup and multi-hit.
And that is a good way to look at Pit's "objectives" during the match. Sure he has some more tools, but those tools above will definitely help you get your foot in the door for this character. Pit is more about punishing your opponent for making mistakes and keep good stage control than getting huge combos, so play with that in mind. He's the "fundamental" character.
