Another note:admittedly, I haven’t learned mesilas yesharim insid, but that being said, I think you find the Rashi discouraging because you’re coverging two different levels together:
Meaning, (if I guess the context correctly), the mesilas yesharim is saying to focus on the pleasures and rewards of next world and make that the focus of our lives so we don’t get carried away by the temporary pleasures of this world and lose track of our mission and then be overwhelmingly embarrassed upstairs…
However that is helpful baby steps in Avodas Hashem because as pirkei avos says we shouldn’t be like those who serve the Master for reward but like those who don’t do it for reward.
Ie.this is a helpful train of thought when we are struggling with an immediate temptation so we don’t fall for it. But the end goal is not to be self servants focused on our own rewards but Hashem’s servants who ask ourselves what does He want from me? And internalise His desire for a G-dly world, as our own, and work to get there to do it for Him out of love and awe. When the focus isn’t on
self service, but on Hashem service, then the Rashi won’t be depressing
Another note: the Rashi may be referring to whoever enters gan Eden right away, which are those who don’t need to be scrubbed clean first in gehennom. The majority of us don’t fall into the truly righteous category and have some cleaning up to do before we can bask and enjoy the Shechina. However, if you are a frum Jew and do your best with the fall here and there, then the clean up isn’t so massive (the gemara gives a time limit I just forget if it’s 3, 9 or 12 months) and then you are free to enjoy the good you have created forever without the nasty reminder of the other stuff…