When Moshiach comes - any day now, G-d willing, we will yearn for the days when we had the struggle, when the struggle was so real. There is no greater connection with Hashem than when a person feels that he needs Hashem every second to save him.
The Sefer Shamati (from the Ba'al Hasulam) explains that the real definition of "Simcha Shel Mitzva" is as follows:Davka when a Jew feels no simcha in serving Hashem and he feels dried out and so far from Hashem, yet he knows what he really wants deep down and instead of letting these feelings stop him, he arouses himself to rejoice in the realization that he "needs" Hashem so much... That is true Simcha Shel Mitzvah and that is the truest connection with Hashem that a person can have.
And the Ba'al Hasulam gives a parable. If someone is dying and only his friend can save him, he will beg his friend to save him with every ounce of strength he has. He will be totally dependent and attached to his friend. However, if he is not dying and he only wants his friend to give him something of "extras", then he will not be so attached to his friend through his desire and request.
When a Jew feels that going back to his old ways is "death itself", in that it will tear him away from the source of "life itself", then it becomes to him a matter of life and death. And when the struggle is hard, he knows only Hashem can save him from this death! And he will not stop needing Hashem and begging Hashem to save him every day. And this is the TRUEST and STRONGEST connection a Jew can have with Hashem, says the Ba'al Hasulam. It is even stronger than the connection of Tzaddikim whom Hashem has already saved from the Yetzer Hara. For such a Tzadik is like someone who got a present from the king, took it and left. But the man who is begging the king to spare his very life, davka BECAUSE it is so hard and he has so little strength, this is a man who is truly davuk (attached) to the king.
Tzadikim who had already beaten the Yetzer Hara were known to have greatly yearned for the days when they still had these battles, and felt so dependent - and therefore attached - to Hashem. As the Pasuk says "Me'rachok Nireh Li Hashem" - "from afar Hashem appeared to me". It is the feeling of being far, that in reality brings us so close. And this should give a person the greatest joy, ironically, davka at the time he feels no joy - because in such a state he has no one to rely on but Hashem.