It's always better to talk about breaking up a relationship. A telephone conversation is acceptable in some cases, but still there is an element of understatement after it, only personal presence can dot all the i's and convince your partner of the finality of your decision. But if the conflict in your relationship is so insoluble that you cannot even see your ex-lover, all that remains is to break up with him via SMS.
Make sure there is no other way. Parting by SMS is an extreme measure, which is better not to resort at all. In addition, the young man may take the message for a practical joke or think that your phone has fallen into the wrong hands.
The message does not have to be short to monosyllabic, but the abundance of words you do not replace your own presence. On the contrary, the young man decides that you are convincing yourself of the need for a break, which means that you doubt. Therefore, choose a concise wording. Write it down on paper and try to read it through your boyfriend's eyes. Consider his condition: angry, depressed, confused.
Express yourself very clearly and concisely. Do not ask for a "time-out", do not take an adjournment.
Possible wording: “I'll take my things on the weekend”, “It's all over between us. I'm sorry”,“I don't love you anymore”- and so on. The context of the SMS correspondence does not imply an explanation of the reasons or a clarification of the relationship. You are simply stating a fact.