is it considered baal tashchis to throw out leftovers? (like from shabbos?) or do you have to force yourself to eat the leftover scraps of food?

ב"ה Shalom, Bal Tahschit is a negative commandment from the Torah. Though some commentators among the Rishonim say the Torah prohibition applies only to the destruction of fruit trees, (Rambam Melachim 6:8,10) there are those who say that the pointless waste of foods and other objects is also a Torah prohibition. (See: Sefer Hachinuch 529, Semag Lavin 229, also see Talmudic Encyclopedia Vol. III- "Bal Taschit"). Before I answer your question directly, I feel in there is a need to address the issue of wasting food since in our times when we live in a world of abundance. The Sefer Hachinuch writes that the way of the pious should be that "not even a mustard seed" should go to waste and one should feel sad for any unnecessary waste . Rashi (Taa'nit 20b) writes that if one wastes food it is as if he despises the gift of Hashem to this world. I think this gives us some perspective of the attitude of our Rabbis to Bal Tashchit. Perhaps it is also appropriate to add here what I heard once many years ago from Rav Yisrael Meir Lau Shlit"a, former Chief Rabbi of Israel and holocaust survivor, who relates of his urge to bend own and pick up even a dirty stepped potato upon seeing one in the street even as Chief Rabbi. Now to your question. When preparing for Shabbat, one should prepare things accordingly so that with all our good intentions of preparing nice food for Shabbat, we should not prepare over abundantly if things will not be eaten and end up in the waste basket. If possible, left overs that can be kept, can be put in the fridge or freezer to be eaten at another time, or perhaps arrange to have it given to a needy family. This would be the ideal situation. However, many Rabbis have said that if all the fuss around saving the food is time and energy consuming, one is not required in making the effort to do all the food saving. So, if you have leftovers and there is no one to eat the food it may be discarded. This is because our main concern is that food not be wasted at of pure disrespect and disregard for food. However, if the food will not be eaten and in any case will go to waste we can find justification in discarding it. (Piskei Teshuvot 171: 3, She'lat Shlomo vol. II. 54) In any case, it is improper to gorge oneself, so people need not force themselves to eat what they cannot. It is acceptable and it even was once a more practiced custom to leave something over on one's plate so as not to look as one eating out of voraciousness.(Piskei Teshuvot 170: 4). All the best and a healthy winter.