We use past tense forms to talk about wishes, even if the consequences are in the present, so the second sentence is better. Strictly speaking, you should use were rather than was, although in everyday usage was is nearly as common.
I wish it were true that I didn't love you.
"I wish it were true" is a way of disagreeing with something that somebody has just said, so it is appropriate if the other person has just said "You don't love me".
If this isn't part of a conversation and you are simply expressing a wish, you would probably say
I wish that I didn't love you.