생각했고 --> did think and --> thought and ...... 했 is the past form of 하다. and 고 is the connecting suffix. so It's like I "thought" 사랑이라 well,,,It could have so many different meanings and feelings I think I should read whole sentence. Sorry

oh,, 내 사랑이라 생각했고 It is (나는 너가) 내 사랑이라 생각했고 means I thought you are my love. in this sentence (나는 너가) is missing. but we can understand without saying it. yes, sometimes both Subjective and Objective are missing but still it is understandable.

"했고 and 이라" 했고: 했다 (past form of 하다 (do)) + -고 (verb connective meaning "and"). 이라: shortening of "이라고" 이라고: 이다(is) + -라고(connective indicating a quotation; (the fact) that...) 사랑이라=사랑이라고: that it is love 내 사랑이라 생각했고: (I) Thought that (you/he/she/...) were my love and ... You can imagine a full sentence like: 나는 네가 내 사랑이라(고) 생각했고 그 생각은 변하지 않았다. => I thought that you were my love and did not changed my mind about it.