You write that you suffer from talking before thinking, which means you are not being thoughtful.
The teaching of the Alter Rebbe is well-known regarding the verse "you shall quickly eradicate the unwanted elements from the good land of Israel which G-d gives you". The Alter Rebbe explained the verse in another way: One is to abolish "mehera" - hastiness and impatience - the opposite of thoughtfulness. When this is accomplished, the "good land", spiritually speaking, is revealed.
Practically, this means you are to study calmly and patiently, enjoying the essence within your studies. You need to eliminate impatience from your approach to learning and life. When you dwell on the Alter Rebbe's words, it will correct your problem, enabling you to learn with thoughtfulness.
The Rebbe then referred to a talk from his father-in-law [Sefer HaSichos 1940 p.59] which documents the original story with the Alter Rebbe. The Previous Rebbe recounted the story as follows:
Rabbi Yosef Binyomin relates how his grandfather, Reb Elye Reuven, once sat and studied in the beis medrash of Reb Moshe. He learned with extraordinary diligence, out loud and very quickly.
"I understood well what I learned, but learned very fast. The Alter Rebbe was sitting in the library of the study hall. As he passed by, he told me I was learning with much passion, but too quickly. I told the Alter Rebbe I was by nature an impatient person. The Alter Rebbe replied "One needs to change one's character".
I retorted "I cannot change my nature".
The Alter Rebbe responded "A Jew has soul powers and can change his nature by accepting the yoke of G-d. If one practices making the change, ultimately it becomes second nature, and the newly acquired nature transforms the original nature. Kabbalas ol (accepting the yoke of G-d) is a fundamental teaching in Torah and serving G-d."
Reb Elye Reuven said "The Alter Rebbe told me that the passion with which I learned was a gift from G-d. The Torah says "v'avadetem mehera meial haoretz hatova asher Hashem nosen lachem". The word "haoretz" also means "willpower", derived from the root word "ratzon" and this means that that willpower to learn Torah with passion comes from G-d. However, the Torah cautions one to use that passion for learning Torah by enjoying the spiritual delight of Torah. This can only be felt if one eradicates the hastiness from his learning. This is the meaning of the words "v'avadetem mehera": Get rid of "mehera" (hastiness and impatience)".
"The Alter Rebbe, with his few words, eliminated my impatience and changed my nature. My hastiness was removed and substituted with patience, which made me fortunate for the rest of my life."
Source: Igros Kodesh of the Lubavitcher Rebbe, vol. 9, letter 2923
"The Rebbe's Advice" adapted by Rabbi Chaim Dalfin