Even though I told him that I hate him so much.
I understand most of it except for しまった。Looks like a verb in past form, しまる? 閉まる? That doesn't seem to make sense.
I believe it's a past form of しまう. In this case it adds a sense of regret to the phrase. ("He sent me a present... And I told him that I hate him before... That was so wrong of me!") Here is the first article on the topic that popped up in Google: Using ~てしまう to express regret or completion of something.