I agree with puffin, AndreasS, TM, Mary, and Prof. Brians (Mary's link) that in standard written English, 'as if' (or 'as though') takes the subjunctive and 'like' should not be used as a conjunction.
As we've established in several previous discussions (check the archive, under 'SUBjunctive' and 'KONjunktiv' BTW), I don't agree with Ghol that
>>In normal English we tend to avoid the subjunctives where possible.
As ThomasJ mentioned, modern BE does apparently use the subjunctive less than modern AE, particularly in conversation. But on both sides of the pond, careful writers still use it, and learners of English definitely need to learn it.
So the standard, correct written sentence is
• I feel as if I had two personalities. (formal, subjunctive, contrary to fact, emphasizing that I know I don't really have two personalities)
The normal colloquial spoken sentence is
• I feel like I have two personalities. (informal, indicative, emphasizing that I really do feel this way, that the feeling is real and present to me)
For me, the in-between version sounds awkward because it mixes formal and informal diction:
• I feel as if I have two personalities. (formal 'as if,' colloquial 'have')
However, I basically agree with Mary and TM that in both cases, the indicative ('have') simply reflects a difference in emphasis: The present feeling is experienced more as a reality, a given, than an irreality, a hypothesis.
Mary's explanations are all helpful, and I further agree with her on this point:
>>"I feel as if I have wasted all my time." - I can't imagine a situation where this would require the subjunctive. You believe
>>that you have wasted your time. It's a real situation.
However, like AndreasS, I would go further and say that it might be more normal without 'as if':
• I feel (that) I have wasted all my time.
But the subjunctive simply isn't used after 'like,' so this sentence isn't English:
>>*"Sometimes I feel like I had two personalities, but I'm really not sure about that."
Perhaps one way of explaining it is that 'as if' can feel either factual or hypothetical, but 'like' always feels immediate, real, without the distance created by the formal conjunction and the subjunctive. Would it help to think of 'I feel like' [coll.] as simply meaning 'I feel (that)'?