Here are a few basic things to keep in mind when you practice loving kindness meditation:
First, most people find this much harder than concentrative meditation. Try not to get discouraged and keep at it - it gets better. One thing to keep in mind is that Buddhist meditation is about self improvement. If you are having trouble with a technique, this suggests it's a weakness, which means you need to work on it more, not less. Sometimes the ay it's difficult is interesting. It's not uncommon, for example, for people to report they have a hard time doing meditating love towards themselves, which suggests they need to work on being less judgmental about themselves, etc., etc. In any event, try not to indulge your whims by avoiding meditation techniques you don't enjoy or aren't good at, because then you will never improve in these areas you need help on. My son does martial arts and he hates doing kicks with his left leg, because it is so much worse than his right. But I tell him this is precisely why he has to make himself work the left...
Second, you are not trying to actually bring about these emotional states in other people - sometimes practitioners will get all metaphysical and talk about actually influencing others. Personally, I don't think this is possible, at least not directly, and in any event it kinda misses the point. The point is to improve YOU. It may be that if you can make yourself a better person it will ultimately help others, but while you are meditating, you are working on your emotions.
Third, it's often helpful (for lots of meditations) to visualize something. This is fine as long as the visualization doesn't become a form of monkey mind itself and lead you astray. Keep it simple is the basic rule. For example, you might imagine the face of the person you are thinking about and then imagine them smiling as you wish them well.
Now, let's do another 20 minutes of loving kindness - first yourself, then a friend, then a neutral person and finally a difficult person. Be sure to use different people than you used last time as the idea is to learn to generate these feelings about anyone.