okay, so the first thing to remember is basically a cliche at this point, but it's also true - you can't edit an empty page. beginning stories is hard! i rarely edit extensively after finishing the first draft, but one of the things i used to edit the most when i had a bit less experience were the opening passages. i have since learned to determine how i want to open with a bit more precision, but it was definitely a learning curve, so don't be discouraged! even if you don't like your original beginning and feel that it's not quite right for the story, you can edit it after you're done with the rest of it. but if you get stuck on the beginning, the rest of the story won't get written. (and honestly, the same goes for any other part of the story - you can edit it after you're done and you can ask a friend to read it so you have a second pair of eyes/second opinion before you publish, so you know what to improve, but it has to be there on the page first.)

it's great that you have a general idea of what you want to write, but make sure that you know where you want it to end - especially when it comes to the characters' mental and emotional state. where do you want them to end up? what kind of emotional space are they moving towards? are they running from something? are they looking for something? and in the end, do they confront that need to run away? do they find what they're looking for? answering those questions can help a lot in developing your characters and their relationships with other characters. also, if you think the development is lacking, there might be many reasons for this - try to figure out what aspects of that character/plot development are not working. is the pacing too slow? too fast? are you leaving enough breathing room for the characters to react to the events and respond on an emotional level? are you making sure that your characters are not just stuck in one place emotionally for long periods of time? all of those things are factors that can contribute to the impression that there's something off with the development of the characters/plot/main pairing.

above all, though, don't get discouraged! writing is a skill that can be only learned through trying and looking at your work with a critical eye, and finding areas to improve! there are, of course, people who can help with that critical look, and finding a good beta-reader is a great way to make this process more effective, but even having a friend read through the story and share their general opinion can be helpful.

good luck with your writing, anon, and let me know how it goes!