Okay, breathe. Relax. Google Calendar is your friend.
It is possible to start two relationships at once, as long as you're not putting their needs before your own. Don't say yes when you mean no. Tell the truth. You can let someone know that you can't see them because you are busy, or that you need some time alone -- a friend of mine says, "I'm taking a mental health day." Don't feel like you have to hide when you want a day or evening by yourself or with your platonic friends.
I think, for me, I would look at my week and the commitments I have to myself and prioritize those first (like school, work, family), and then figure out how often I can see the people I'm dating and fit them into the other days/nights around my schedule. It might be that one person can see you twice a week, and another only once a week, and there's nothing wrong with that.
As far as wanting your bf to slow down with his other gf, is this because you don't feel like you have his full attention when you're together? There are ways to make sure you are both investing your energies in cultivating the relationship you have - first, by talking, and also by making requests for things that would make you more comfortable with it, like no texting the gf on dates with you, for example.
Really, though, I think if you schedule time for yourself and make sure you aren't spread too thin, that will take care of a lot of issues.