HI HollyGolightly, I did have this problem when I was younger. surprise though, it is actually something you can work on and vastly improve. i would say that i have become very comfortable being extremely accurate and articulate in expressing myself, both in writing and verbally (in certain circumstances) almost all the time. journalling helped me with expressing myself in writing. that is something which felt natural to me. expressing myself verbally, especially in person is a different story. at this point in time, i feel confident doing this, but this took many painful conversations and painstakingly working on this skill. but it is just that - a skill, and you can improve it.
when i was in high school, i was at times vastly shy and had very little confidence with confrontation. sadly, i have had to have many difficult and painful conversations in my life. at first, it helped me a lot to write down some of my thoughts or the things i wanted to talk about in dot points before i had a difficult conversation with someone. being INFJ, no matter how much i hated having these difficult, emotional conversations, if it was the right thing to do, i had to do them regardless. the more of these difficult conversations i forced myself to face - always with honesty, integrity, and forcing myself to say exactly what i really mean - the more confident i became in communicating (both difficult conversations and normal ones).
i still ramble sometimes, but i also noticed that i would do this because i didn't know exactly what to say, or more often, i felt pressured to answer immediately. in time i have learned to spot when this happens and allow myself to be quiet and think in silence until i am ready and comfortable enough to answer. one big thing that effects my ability to articulate myself is how calm or comfortable i feel. i have learned that personally, if i am very tense or stressed, pushing through that first block of getting a difficult conversation started means i will be able to push forward and get the rest of how i feel out.
i swing between rambling away (mostly to family or in situations where i just get carried away by something - this embarrasses me when i do it in front of people i don't fully trust or know yet), being quiet, and being extremely succinct and well expressed. rambling (when i can't seem to shut up) is a warning sign to myself that i am uncomfortable in a situation, or there is something i should be aware of (stress, etc). contrary to what seems to be the general agreement here, anyone who i have shared my thoughts or writing with have said that i am extremely articulate - however, this only happens when what i have said to them or given them to read has had enough time for me to form my thoughts calmly and carefully.
take your time to think about what you want to say, and try not to let your surroundings or other people put pressure on you when you are trying to formulate your thoughts. you are allowed to think carefully before you answer, don't feel the need to apologize for that. don't worry about struggling with expression - it will come naturally with practice and time.