How can I always maintain a strong and intense aspiration?
The mistake that you and others make is that you have a fixed goal. If you come to a certain standard during your meditation, you feel that you have reached your goal. Or if you get a little joy in your inner life, then immediately you get a complacent feeling. You want to rest on yesterday's laurels. But I wish to say that our goal is an ever-transcending goal. Yesterday you got an iota of joy, and today you are crying to get that same iota of joy. But how do you know whether the Supreme wants you to have that iota of joy or whether He wants you go farther, higher and deeper?
In your case it happens that you always try to reach a particular goal. If you know how to run fifty metres, then after you have run fifty metres you feel that your part is over. But me Supreme does not want you to be satisfied with fifty metres. He wants you to run fifty-one, fifty-two, fifty-three, fifty-four metres. When you have a higher goal, automatically your aspiration increases. Otherwise, if you always aim at the same goal, you don't make progress, and it becomes monotonous. If you always go to the same place, after a while you don't want to go there anymore. But if you feel that your goal is constantly farther, higher, deeper, that it is ever-increasing, ever-ascending, then there is constant enthusiasm.
Joy is in progress, not in success. Success ends our journey, but progress has no end. When you have a fixed goal and you reach it, that is your success. After that, you are finished. But if you don't have a fixed goal, if your goal is going higher all the time, then you will constantly make progress, and you will get the greatest satisfaction.
So do not be satisfied with success. Aspire only for progress. Each time you make progress, that is your real success. Every day when you meditate, feel that you will go still deeper, fly still higher. Then you will be able to maintain your intensity and enthusiasm.