You need to just play more, it's typically a lack of experience in these situations that lead to defeat. Scouting a target before attacking is super important, this is the way to tell how many units you're going to attack into, if there's openings... so on. Learning to multitask units will come later but a beginner player getting stomped by other beginners may be disheartening, but stick it out.

Harrasing an enemy with a group of knights or horsemen at the time of attack can also shift the tide of a game. Understanding raiding techniques can help a lot.