I can definitely sympathize with a lot of what you're feeling now. I have been there myself in the past.
I know you said you can't access you school's psychiatrist because it is summer, but could you possibly find another one? Maybe you could check with student services to see if they do offer some sort of care in the 'off season' or if they could suggest another doc or therapist who could work with you. At the very least, having someone you can talk to who *legally* can't blab about what you share with them can be very freeing and it's good to have a professional to help guide you through the process of figuring yourself out and accepting yourself fully. It's not about finding a doc to "fix" you, but rather someone to support and guide you. It can also be helpful to speak to a psychiatrist to rule out any possible chemical imbalances, as GalaGirl mentioned. I'm not suggesting that this must be an issue for you, just that it could be a possibility and treating an underlying condition can help make things much easier when it comes to self-esteem, abandonment issues, and relationship issues. Sometimes you need a little assistance to get your thinking back on track and see all the possibilities. (I have firsthand experience with this as well and would be more than happy to talk privately about any of it if you'd like.)
I definitely agree with the comment about being overly fluid, like water taking on the shape of its container. I have described myself that way at times. It's a slippery slope and makes it easy to 'lose yourself' in the process of trying to adapt to other people. Total reliance on other people's happiness and approval for your own positive feelings is not healthy. You mention feeling responsible for your ex's well-being and other people's emotions as well as feeling like you "need" your partner and rely on them like an authority figure. These things (and a few others) definitely throw up a bit of a red flag to me that you may be experiencing some codependency issues. It might be worth looking into some things on that topic. Melody Beattie has some great books about letting go of the overwhelming *need* for other people and learning to focus on being there for yourself first, which allows you to really be present with other people and appreciate your relationships for what they are, not to have to force them or use them to meet all your needs. (Again, this is an issue I have personal experience with and would be more than happy to talk about more if you'd like.)
Along the same vein, you mentioned that as far as your time goes, if you aren't with one partner you would be with the other. The concern here is, where is your "me time"? Do you feel you must be with a partner at all times? Some people need more time than others outside of their relationships but everyone should have a little breathing room from time to time. As tempting as it may be, especially in multi-partner situations where time is an important and finite asset, it's just not healthy to need to be with a partner (one or more than one) at all times. It can also be very taxing on you partner(s) to feel they must be there constantly and lose their individuality or personal time. Rather than just being together and close, people can end up enmeshed and unable to distinguish their own value outside the relationship or their wants and needs from the other person.
There are certain things that you should know your boundaries on and not compromise. You definitely shouldn't feel pressured into having children if you feel you are not ready or don't want to just because you assume it's what you have to do or it's what someone else wants. And you have to live by your own time table, not assume you must have kids by 30 or you have failed someone.
It is tempting to want to please everybody all the time but it's just not possible. You have to figure out where you stand on things and what you need, and be able to value yourself separate from your worth in a relationship before you can really bring all you have to offer into a healthy relationship. Trying to please your partner all the time is not fair to you or to them. It's all about finding balance. You can't expect a relationship or another person (or people) to provide you with all of your self worth and value and to meet your every need. It's just not possible or healthy.