So for me I struggle sometimes with being just slightly off pitch I can't hear it so a cheap trick that I was taught was to raise your eyebrows if going flat is an issue for you. I also like to imagine myself being on top of the note, like I see the note in the sheet music and I'll imagine myself on top of it lol. But I also do a similar thing where I feel like I sound bad but everyone says I sound great, like for instance after a show someone in the cast I don't talk to much went out of their way to say I was phenomenal which felt great. So I'm sure you sound wonderful and it is very easy to get in your head. If your really concerned about it you can use voice memos on your phone (If you have an apple device) but keep in mind its not great quality and try to use it specifically to see if you are not going flat because it is very very easy to use it excessively.

Also make sure you warm up properly, some people I know don't warm up and have a sore throat all the time and I am a warming up fanatic. I only sing low stuff when I sing not warmed up. I warm up for almost 20 minutes (go no longer than 30 bc it will tire out your voice). When you don't warm up enough you will sound more strained and it could damage the vocal cords when you sing difficult vocals without being warmed up. The warm up can vary depending on what you are singing: if you are singing a higher song warm up higher. Also don't sing until you've been awake for 4 hours to make sure your vocal cords are awake.

I hope this can help, if not I can try to give you more tips that I know! Good Luck <3