Lately I've been waking up earlier, but with no change of bedtime. I have paid the price for this change in loss of concentration, mistakes in my writing, and diminished comprehension of my lessons. On top of that, I've had headaches and felt tired or forgetful all day long... I suppose if I kept sleeping this way, I might get use to it some day. However, for the time being I choose to play it safe and return to my normal sleep routine. You need to say "kept" instead of "keep". This is "past subjunctive mood". (It is not past tense.). The reason you need to use it is that you are imagining a hypothetical situation. Past subjunctive is almost always followed by one of the modal verbs: "would", "could", "might", or "should". In this example, "might" is the one that works the best.

Lately, I have tried to get up early without going to bed early. The result is that I find it hard to concentrate, I made lots of mistakes when writing and I found it difficult to understand a lesson. Besides, I had a headache and felt tired or forgetful all day. I think if I try to keep sleeping less like this, I will get used to it some day. However, I have chosen to return to my normal sleeping routine .

Lately, I have tried to get up early without going to bed early. As a result, I felt it was hard for me to concentrate, I made lots of mistakes when writing and I found it difficult to understand a lesson. Besides, I had a headache and felt tired or forgetful all day... I think if I try to keep sleeping less like this, I will get used to it some day but for now, I'll come back to my normal sleeping habits.

Lately, I have tried to get up early without going to bed early. And the result is that I was unable to concentrate, I made lots of mistakes when writing and I found it difficult to understand a lesson. In addition/what's more, I had a headache and felt tired or forgetful all day... I think if I try to keep sleeping less like this, I will get use to it some day. However, at this time I choose to come back to my normal routine of sleeping.