Been Told by My Close Girl Friends that I am maybe one of the densest people they know

I am a introverted guy in my 3rd year of college now and I have an awful hard time reading girl's signals. It has been this way for a while I can't tell if they are just being friendly or flirting. Although I only know of one time in my life where I was super dense which occurred in highschool after talking about it with my close girl friends who saw the interactions go down and talking about it just recently when meeting up.

Basically in the 10th grade there was this girl in my class who would say hey to me everyday in class in a very elongated weird way and around school and then as time progressed on she was starting to ask for my jacket which I always wore since I felt insecure about my body for the longest. Feel a bit better about it now being a tall overweight guy but am still working on it. Anyways I felt bad saying no I only did maybe once or twice since I would overheat pretty easy in class but the days I said no I told her I legitimately felt cold so I needed it while feeling bad saying no. I saw nothing wrong with it her asking for it although no girl had ever asked me to borrow it befofe. One day she gives it back to me after class and runs off and I notice something in a pocket like an origami heart. I ask my friends and they are like dude I think she likes you and I am like nah no way it is probably meant for some else and ignored it and left it in the pocket so she could take it back the next time she asked for it. I didn't feel the same way about her I thought she was nice but didn't actually know she liked me till looking back at this years later with my friends feeling like an idiot. It was a crisis averted in a way since I then later dated someone else in high school miraculously and this girl I missed the signals from flamed my then gf now ex without me ever knowing till near the end of the relationship so turned out she was an awful person so glad didn't fall for that.

In short I need to figure how to be less dense and learn how let others know how I feel when I catch feelings I just get scared of losing good friendships and being avoided but I guess if they do that it shows they aren't the best people since once had a close highschool friend ignore me for years till I graduated and revisited when she learned I liked her from my friend who told her so she ignored me always then felt crushing to lose a close friend.

Sorry for all the rambling and side thoughts I have a hard time writing out my thoughts.