I would appreciate your feedback on some thoughts that have been bombarding my mind and stressing me for quite some time now.

I am a double major, junior undergrad who is planning to take the LSAT sometime in the future. I will certainly be taking it in September this year. My first dilemma is whether I should take another one in June or in December so at least I can get the experience. Currently my school schedule has been consuming all my time (not kidding). I have barely any time to study for the LSAT and I do not feel confident with my knowledge on the test. I read the KAPLAN Premier 2015 when I have the time in order to get to understand the various question types of the test. I took only one diagnostic by myself during the winter and while I didn't time myself harshly, I got a 139. Since that day my mind is a nightmare. I keep stressing about how am I going to do this. I currently have a 3.7 GPA, not including my transfer GPA which is higher (3.83). I think I have the potential to reach a high score but distancing my study sessions so long is inefficient and is not preparing me for June, The more I wait to apply though, the smaller will become my chances for admission...I dream of T14 - Harvard and Cornell in particular - like many of you aspiring law students. I admire those of you who made it to such law schools. I am the oldest of an immigrant family and I want to do the best I can to help my family and become something in this society (of course by pursuing a career that interests me), but I feel that this dream is slowly diminishing with having no time to prep and getting an AWFULLY SAD diagnostic. In order to graduate on time I will have to take a may term course and one or two summer courses. My second dilemma is if from your experience taking summer classes is possible with LSAT studying. I do not know how heavy is the workload (Spanish classes, but non-Spanish native), and I want to make the LSAT my priority. I plan on quitting my job until the fall for this reason. Another possibility is two graduate next summer rather than in the spring.
Do you think I have a chance of hitting 170? At least past 165 in 3-4 months? I have to admit, LG are really hard. I am not used to this mode of thinking. I am bad with figuring out what needs to be deduced. I hope this will come with practice.
Should I postpone taking the LSAT until December?
I also would like to sign-up for a Testmasters course during the summer. Would that be of help?
From your experience, should I quit KAPLAN and just start doing practice exams questions?
I know I have written a lot and I humbly apologize if this post has been made in the past.
P.S could my low LSAT score have been due to being a bad standardized test taker? My ACT was 18 and 21, but my GPA does not reflect that accordingly.
THANK YOU SO MUCH in advance!