First off, I think you deserve a bit of credit for being quite honest about your situation and how things got to where they have. It's not the easiest of things to talk about when you've had knock-backs like you have, so fair play to you for giving a comprehensive account of your experience.
With that being said, what I'm going to say probably isn't what you're expecting to hear but I think it's worth bearing in mind anyway. It's this: I think you need to take a step back and have a proper think about what you're planning to do and what you expect to get out of it. I understand all too well why you would feel this is the best approach to take, as it sounds like you've gone through a fair amount of pain (or at least turmoil) over Sarah and to a lesser extent what happened with Gia beforehand as well. However, in light of the fact that before you made a move towards a relationship you and Sarah got on very well by the sounds of it, I would question whether cutting her out of your life completely is the best approach. Given that you mentioned that she is very focused on her studies and doing the best she can with that, and also having issues of her own outside of class regarding her body, it could very well be the case that she just isn't ready for a relationship and that you hinting at wanting one may have scared her off. People seldom start avoiding each other without there being a reason behind it, particularly in such circumstances. Outside of you pursuing her for the purposes of a relationship, it sounds like the two of you have a lot of common ground. Is that in itself not worth holding on to, even if for now it is only in terms of friendship? You still have a few more years in school, and it isn't beyond the realms of possibility that in that time you could rebuild your friendship, and from that something more may happen. Or it may not - who can say? But given what you said about finding it difficult to have friends, or certainly close ones, you need to be sure that getting rid of someone who could potentially be a good friend if nothing else is the way forward. As things stand, you still have other options so think carefully.
Aside from that, I would say the main thing in your situation is to look after yourself and make sure you're not putting yourself in a position to get burned again with the Russian girls you mention. In light of what has happened with both Gia and Sarah, I would recommend that you make sure you understand their intentions and that they are genuinely interested in you and don't rush into anything. In particular, I would suggest you be entirely honest with yourself and be sure that getting into a dating/relationship situation is one you are entirely comfortable with - and I say that because, in the nicest possible way, it does sound like these things affect you to a greater extent than might normally be the case. I don't presume to know what is going on in your life beyond what you posted, nor about your wellbeing at this moment in time, but that is what comes across so again I would stress looking after yourself first. At the risk of sounding like a patronising old fart well beyond my 26 years, you are still young and there will be plenty of opportunities to date girls and have girlfriends without feeling like you
need to be in a relationship - which, in some respects, is how it comes across at the moment. I can appreciate that, having been single throughout high school while others were dating, but you should want to be dating that person because it's that person rather than as a "backup plan" or similar.
Sorry if that isn't quite the reply you were looking for, but hopefully what I've said makes sense. If you'd like to discuss things further, please feel free to send me a
PM and I'll do my best to help.