I know we will be able to meet next year. (So,my words may sound too sentimantaly.) But I WANT say that...... See you soon.I'm going to miss you! (I think he told you he feel lonely without you even if short period.(or even if promise next meeting)) --- 承知の上で～する。 知った上で～する。 分かった上で～する。 ex)無理を承知の上で、お願いする。 I fully understand that I’m asking you to do too much, but please do let me ask you this.

来年、またかならず会えるということを知っている上で伝えるよ。 ((その後英語で)) See you soon, I'm going to miss you!」 I am going to tell you this now because I know and believe we will meet next year for sure, It sounds like that he doesn't want to say good bye to you, by saying see you soon, which means good bye.

Next year , I will tell you that we can meet again till It is assured .(こんな感じ)

Next year, I'll tell you with knowing that we'll be able to meet.　こんな感じですかね…？