Feedback is super important whether it's your own DJ mixes, remixes, or tracks. One of the best things you can do is give your project the car test, or listen to it later on with a critical ear on headphones or someplace other than where you created it.
If it's something you still feel good about then post it online for others to listen to. You can simply just post it, or you can post it and ask for feedback.
And this is where it gets tricky.
You may get good feedback or you might get overly critical non-sense from people who aren't even on the level you are. Then there's those who can't give objective feedback because they only see through their own viewpoint.
Some things to consider.
When you're submitting demos remember that the people who may give you feedback such as your friends or people on Reddit and forums are NOT the people in A&R at the label. In the end feedback from your target audience is all that matters.
So using sending a demo as an example, don't ask random people, "What do you think of my latest track?" This will just open you up to receiving a bunch of mostly worthless opinions. Instead, ask people who like the same kind of music as you do, "Do you think my latest track fits the sound of ______label?"
When you ask better questions and also target a specific group you're far more likely to get useful feedback.
And even then take that feedback with a grain of salt because those people are not the decision makers - the ones who can say yes or no to signing your track, remix, or guest mix.
I asked someone very close to Victor Calderone about my remix submission last year. She's also in the biz and knows music. She didn't think I did enough to put my own stamp on it. I could have let this put me into a tail spin of changing things back and forth and likely wouldn't have won. But I liked what I did and wasn't going to get paralyzed by feedback.
Also another producer friend liked it but didn't give it 2 thumbs up. So until you really have someone's ears you trust - don't get hung up on what outsiders say. They may have valid points but they aren't the decision makers. They can say no, they can say they don't like it, it's not a good idea...they can tell you all these things some good, some bad. But in the end they can't say yes.