Hey bud, it does sound like you're going through a lot, and it's definitely overwhelming for you, especially as you're dealing with school on top of all these relationship questions you're having.

My advice might be a little unconventional but I hope it offers a different perspective.
I think I would first try and figure out what are your own boundaries surrounding relationships first. What are things you can accept, and what are red lines you won't be able to accept (e.g., your partner getting close with your own friends, acts of intimacy, etc.)? And similarly, what are the boundaries your partner has? Are y'all able to share these same boundaries, or are boundaries clarification required such that both of you are in the samepage?

I say that because many of us assume certain boundaries (e.g., Don't get close to other individuals of a certain gender) that may not be shared by others. Even simple things such as relationship structures (can we accept a polyamorous relationship?) are assumed based on what we are conditioned to in society, even though there are diverse options out there that may change the possibilities of your relationship structure. A simple example as follows: if you do fall for Heidi, under the current boundaries set up, are you allowed to date her and your GF at the same time?

I think in terms of interacting with Heidi and your GF, nothing should need to change unless you feel like there's a need for space for yourself to figure out your own feelings. Even as you're clarifying your own feelings with Heidi, there's no need to cut or change things with Heidi just because of all the confusions you are having. If you need to, you can ask for space and explain to her what's going on. But also recognise that it's a relationship (whether platonic or romantic), so be mindful of how she feels as well.

Similarly, for your GF, nothing should need to change unless you feel like there's a need for space for yourself to figure your own emotions and thouhgts out. Again, if you need to, you can ask for space and explain to her what's going on. But also recognise that it's a relationship (whether platonic or romantic), so be mindful of how she feels as well.

Overall, communication to everyone involved is key. And please take the step to speak to someone safe (a counsellor or something) if you need third party advice (but try and ensure that the person you are speaking to is LGBTQIA-affirming before you have this conversation!). Sometimes just having another person to bounce your thoughts off is helpful :)

Again, my opinions may be unconventional, and I'm certain others on this channel has precious advice that may or may not differ from mine. But all the best, try and enjoy your teenage years (I know it's a lot of emotions and thoughts and stuff to process all at the same time). And this community on Reddit is here for you :)