Literally, to force something into a suitcase (for example) would be to push and push to squeeze clothing into the suitcase. This is despite the clothing not being properly packed to fit inside the suitcase's size.
Figuratively, to force something is to attempt to succeed by effort where an action is not appropriate, suitable, or comfortable.
Example: Violet is planning to visit her sister Maggie, and Maggie's husband, Joe. Violet loves her sister but despises Joe, holding great contempt for him. Violet's friend Sarah says to Violet, "You shouldn't force yourself to be nice to him. He's a jerk." Violet replies, "Yes, but he's married to my sister. I don't want to cause trouble for her."
As in this example, people can force themselves figuratively to do something, or attempt to do something, that is against their natures and their desires, either for their own sakes or the sakes of others.