There is a lot here, and you deserve a full response. Here is one! :o)
While I appreciate that this is a serious question concerning an issue that is rightly important to you, we older types often wish we hadn't taken ourselves and most things quite so very seriously during our short teen years, because they are a period of emotional change and discovery – so much changes (as you are recognising)! We can’t avoid changing, but we CAN make decisions about which direction we wish to go in. The important things are to develop self respect and emotional self sufficiency (i.e. not relying too much on someone else for our happiness) and to work on our values and qualities such as integrity and respect for others: absolutely everyone has things about them you don’t know about. Also ensure that you have no unnecessary fear (False Evidence Appearing Real!). We CAN decide and plan about our directions by practising in our minds the kind of person we wish and intend to be. Especially first and last thing in the day when the subconscious is most accepting of your conscious thoughts. (So avoid negative t.v., video games, etc. at night).
Life is full of ups and downs, isn't it. The trick is to take charge of your emotions, and not to allow yourself to get too high or too low. Some people find this easier than others.
People tend to respect those who walk tall, whatever their height, so always ensure you have a straight back, whether sitting, standing or walking. This also helps our self esteem.
Life goes in seven year cycles: infancy till seven, childhood to fourteen and adulthood around 21. Naturally people vary, and the stages merge into each other like the seasons do, but it continues to an extent: many women are not too mature until late 20s and a lot of men don’t even start to grow up until their mid thirties (I was one!).
All parents have been teenagers of course, and can be taken back to their teenage feelings with genuine interest and positivity on your part. That said, no one person can ever completely understand another (except identical twins, perhaps): we all come from slightly different places and have different experiences of and takes on life. Hence nobody can judge us with any accuracy.
Remember: adolescents’ emotions are in a state of flux and constantly changing – some more than others, obviously.
Doing things for others without asking for anything in return, such as voluntary work is very often a key here (what goes around DOES come around eventually).
Of course your mother will miss you, but she knows that you need to stand on your own feet and make your way in the world, so she will also be happy for you. You are mature enough to recognise, as you say, that you over analyse things (I bet not actually EVERYTHING!), and you probably magnify and distort things, too, which is what water does. As you become more mature, this recognition will assist you to balance things up with some down-to-earth practicality, you intellect and your loving heart.
What an exciting period is coming up for you! You'll learn to make new friends at college (email me if you want some tips on how), new experiences, a chance to step out from family life while you are away - but it isn't for ever, of course! New learning, new skills, new ideas........ Wow - I wish I was your age again!! Please try to focus on the positive aspects, but if you can't, at least be open minded. As in, "I wonder how this will pan out", for example. But as I say, you really can plan about how/who you are going to be.
Good Luck, Sunflower! (Great symbol that sunflower - like your heart!)