There are many people who are quite shy and get very nervous when trying to talk to those from the opposite sex. I had a similar problem when I was 12 to 16 years old and remember it being very difficult and even more frustrating. And ironically, when I managed to open my mouth to speak to boys at the time, I always sounded very deliberate, unnatural and said things I never meant to say in the first place, making things even more awkward. At the same time, my classmates seemed to have no social issues at all and were able to establish all sorts of connections. I had to work for years on this but unfortunately I had no guidance at all when I was younger and growing.
Nowadays, you can sign into courses that teach you not only why you are shy, but also how to reverse the mentality, and specific techniques that you work on and apply to establish your new communication skills. Instead of dwindling and being nervous every time you are around girls.
And yes, you need to learn to speak up for yourself. Respectfully yet firmly when necessary
If you'd rather not take an online course on conversation skills and social anxiety management, (and there are several really good ones), then at least buy yourself some books on the subject. Educate yourself, that's all, and then apply your knowledge. But you will have to commit and be consistent. If your shyness truly bothers you, you will find the motivation to dedicate time to this type of learning. Trust me, social anxiety can be overcome by learning new skills and techniques that will calm the part of your nervous system that is raising your adrenaline levels so high that you get paralyzed. it is completely manageable. And the earlier you start learning these techniques, the earlier you will be able to release your own chains.