So you already screwed up
Kissing Amy is a no-no to get the best ending. Yep, just this kiss already, "shit will not thit the fan" because of it, but you will not be able to get the needed 400 points anymore for the very best ending.
Lets say it like this... if you fool around in real life (even if its only kisssing), do you have a rewind button to "undo" things before they hit the fan? I dont think so, if you do... man... make a business out of it and get rich.
There are "points of no return" and if you want to go with a different girl than Melody, you CANT be with melody otherwise... shit -> fan.
If i remember correctly, in week 7 you have to decide if you go for melody or not, after that you have to cut the ties with every other girl... or shit meets fan after a while, but the best ending is ruined at this point anyways already.
On top... if you are not on a melody ONLY route (from the day you meet Melody, you cant have anything going on with any girl, not even a date or you will not get the best possible ending), you cant get the "best" ending, its simply not possible. You have to avoid every other girl period to even have a chance to get those 400 points in the end, there is no other way. Personally the "best ending" felt not that good, to me, there is no "perfect" ending of this game, sadly... there are always some "flaws" in the endings that make them feel a little less good. But yeah, the "best possible ending" is all around the best for melody.
There is one execption to have something with an other girl like melody with zero consequences. You can go for Isabel at the start of the game this "one time" No points will get by you for this... but its the only exception.
I would recommend, play a Melody only route first and after that play around with others... but if you want to start all the routes first and still think you can get the "ideal" ending in the end, you will be dissappointed. You need to get 400 relationship points for the very best ending with melody, and you cant get there if you had anything else with one of the girls, even if its just a kiss. You will lose some points down the way... like, she will ask late ron if you had something with another girl, and if you lie here, she will sense that, and if you tell the truth she will be dissapointed, cause lets face it... fool around with more than one girl is a dick move, especially for a sweet girl like Melody and if you want "pure love" with her... And to make in clear, this here is a "love route game"
The only "thing" you can do, fool around alittle with sophia as Melody, you will get some lesbian scenes only (no threesome and Melody will end things with sophia to be with the MC in the end)
Involve the MC in ANYTHING with another girl, you will ruin the best ending, with no going back.
Ah... and... if you want to get the best possible ending, play with walkthrough, otherwise you will not get there. The game has some really stupid choices where its nearly impossible to figure out how to get the best outcome without the WT. Like... order "chicken or steak" why does it matter? It does, and can ruin the end... really it can. There are voerall 405 Relationship point in the game to get, you need 400 for the best ending... so even some "random" choices can ruin things. And more than once the outcome of a even small decision feels random. To a point where it gets even a little ridiculous... like, you can make some choices for melody and have do denie Sophia to get a lesson with the MC if you want the best ending, even if you dont do anything with Sophia as the MC... so yeah, Best Friend Melody has to be a "bitch" to best friend Sophia cause she doesnt want to share her teacher, if you let Sophia tkae the lessons... no best ending for you, even if nothing happends, cause it opens the "possibility" to be with sophia and Melody doenst like that, even if it was her choice and nothing happens... that is... just stupid. The pointsystem is a real problem of the game, especially if you take a look back at some choices where you think to yourself "wtf is this shit?"
So yeah, i would recommend, use the Walkthrough, stay faithful to Melody if you love her, and enjoy a real nice lovestory... otherwise, the game is a little "meh" the routes for the other girls are not really worth it, lets say it out and clear, the name of the game has a reason, Melody is the ONLY real Loveinterest here, all the others are sidegirls. You dont prefer to be with Melody? Better not play the game... it will feel not good.
If you want to see what is it about with the other girls... just use the gallery unlocker, there is not much more in it than those scenes there...