"I miss you a lot!!" ฉัน - I (intimate/female) คิดถึง - literally, "think about". Here the context is "to miss..." เธอ - you (intimate) มาก - much, very, a lot. This is another common word to look out for. Get to know the Thai alphabet a bit better, and you'll work out fairly easily where "one long word" breaks up into a sentence.