you sitting behind me was less distracting that you came and sat in front of me is this grammatically correct, does this sentence portray what i mean to say if you get it.
No, you're incorrectly using "that" instead of "than" and you're mixing a gerund (sitting) used as a noun with past-tense verbs. You could phrase it like this:
When you sat behind me, I was less distracted than when you came and sat in front of me.
You could also use three gerunds, which would be grammatical, but I find this phrasing more awkward:
Your sitting behind me was less distracting than your coming and sitting in front of me.