You need to get relaxed, put your focus on the space, You can visualize but for FEELING the energy it's not a must. For doing a spiritual work, yes you should visualize, but also put effort on feeling it. Like you would feel happiness, or a touch of others. Get sensitive.
Putting effort on feeling the energy is not straining, but the opposite, being relaxed and dwell in the energy.
Like when you are very thirsty and you start to drink, everyone unintentionally starts to meditate on how good it feels to drink.