That's so exciting! Welcome to the writing community. I am probably the best and also the worst person to come to for this lol. The best because I have a lot of experience with multiple WIPs, and also the worst because I have a lot of experience with multiple WIPs. Here's what I do.
Now, I want to preface this by saying that this is just my system that works for me. Part of writing is discovering what methods work for you and developing your own process along the way, which unfortunately just takes trial and error. I recommend looking into several options to find what fits you best. That being said, I currently have 6-7 active WIPs, meaning I have posted at least one chapter of them, that I intend to finish (and several others I have abandoned indefinitely, which I'll talk about later). I also have 2 big projects in the planning stages as well as a handful of other small projects that I want to do eventually. Plus, I'm participating in the ML Big Bang again this year, so I'm working on a decently small project for that. (Like less than 20k) So, I do know a thing or two about having tons of projects open, and I'm not going to lie and say I handle all of them well all the time, but I've been getting better at managing my projects. Here's some tips that I've picked up over the past few years.
I know, I have been singing their praises a lot lately, and some writers like them, some don't, but seriously, if you're juggling multiple projects, outlines. When you have multiple WIPs, especially for the same fandom or similar concepts, outlines will save you from getting stories mixed up and forgetting things. Even if your outline is just a doc that you dump everything you have in your brain about the story in one block paragraph, do that. Don't rely on your memory of what you want to happen because you will forget. Write it down somewhere, so you have it to come back to when you're more ready to tackle it.
If you're like me and have more ideas than you know what to do with, I usually will dump everything about an idea that I have somewhere be it in a tumblr post or a word doc or a deranged series of rambly messages to my beta. Once I have it written down and out of my head, I can usually assess whether or not it's something I want to actively work on right now. Sometimes we get really excited about a concept because it's really fresh in our mind, and we want to see where it goes. Just writing down a detailed summary or even outlining can be enough to get over that initial impulse to start something new. This can help reduce taking on more wips than you can handle at once and potentially abandoning the one you're trying to work on. If you have an idea written down, you can always come back to it later when you've cleared up some more time and space for it.
I admit, I'm not the best at adhering to a schedule, but it can be helpful to make them. And you can make schedules for various different things like, I want to post this story on this day every week or every month, or I want to work on this idea around this time every day or on a specific day. Schedules can be very helpful at keeping multiple projects organized.
As it stands right now, even though I have a lot of open projects, I'm not actively working on all of them all the time. I tend to pick 2-3 of them to focus on at a time, and even then, that doesn't mean I'm actually writing all of them at the same time. Right now, I am actively writing my BB story (already outlined), my Ladrien enemies AU (already outlined), and I just finished writing KEWK, which is a WIP I've had open for several years that I finally got bothered enough with how long it's been open to finish. I'm also actively outlining My Dearest Buttercup (trying to figure out where the hell I want to take it), as well as a project I haven't announced yet.
Typically, I have one or two stories in the active writing stage, and one (maybe two) in the outlining stage. Outlining helps because it keeps the impulse at bay just enough because it tricks my brain into thinking I'm working on it (I mean, technically you ARE working on it, just not actively drafting). It's also helpful when you actually do get to the point of writing it because you've already made a bunch of executive decisions that could have slowed down your drafting process. Overall, it makes things run smoother.
I try to balance project lengths as well. I try not to take on two big projects at a time, or if I do, I only work on those two, no tiny projects. If I've got one big project open, I may work on a few small projects (one-shots, drabbles) or one medium sized project (like a shorter multi-chapter) along side it. The longer the project, the longer it's going to take you to finish it, so if you have two big projects open, it's going to take you longer to clear those from your plate and allow for time for other projects.
That's why I try to keep smaller projects in rotation while working on bigger things, so I can work through ideas faster and eliminate more things. I tend to have a lot of small ideas that pile up, so it helps ease some guilt to be able to clear those out. If I clear out a bunch of tiny projects, I might add in a bigger project if the first one is nearing completion. It's a delicate balancing act that I've only recently started getting better at. And again, not to sound like a broke record, outlining can help you determine which stories to rotate because you can kind of guestimate how big/long a project is going to be, so you can determine whether you want to take on a big project or focus on a smaller one.
Abandoning stories isn't the end of the world.
I know you expressed fear of abandoning stories, but sometimes it just... happens. I mentioned that I have numerous projects that I've put off indefinitely. Some of them I may go back to eventually, others I may just leave be. Sometimes you move past an idea or a concept or you lose interest in a particular story. It happens, and it's not the end of the world. Sure, some readers might be disappointed if they liked a story, but if your heart isn't in it, then you're not going to be satisfied with it (and usually your readers will be able to tell). Most of the time, my readers are very understanding if I say I don't intend to finish a story (though, it depends on the story, but I've never actively abandoned any extremely popular stories of mine).
It can make you feel really guilty, and it can make you feel like you've failed for giving up on something. But truthfully, writers do it all the time. Inspiration is lost. You realize that maybe a story just isn't working right now. I think for me, I tend to focus on how I've grown as a result of writing that work. Sometimes calling it quits can be a sign that you've grown enough to recognize that what you're working on just isn't for you anymore. Writing is a journey, and sometimes it's better to tread a new path. The act of writing is a labor of love, and if you don't love what you're writing, then you don't have to keep doing it. Allow yourself that kindness, and writing becomes a lot less stressful.
For you, I'd recommend trying the outlining method. If you've got another idea while you're working on something, spit it onto a page in some way, then decide what you want to do with it. Having multiple projects going on isn't a bad thing, and sometimes it's necessary. For my projects, I don't always work on the stories I'm actively drafting all the time. Sometimes I focus more on one than the other. Sometimes I bounce back and forth between them. Just depends.
Hopefully you found any of this helpful. If you have any more questions, feel free to send them!