The Torah instructs us, "You should be wholehearted with Hashem your G-d" (Devarim 18, 13). Rashi explains that one should go with Him unquestioningly and not investigate what the future will bring. Whatever Hashem brings upon a person should be accepted wholeheartedly, and then "he will be with Hashem and be His portion."
Rav Wolbe (Ma'amerei Yemei Ratzon pg. 31) elaborates on this idea. By nature, a person thinks much more about the future than he does about the present. His imagination runs wild with the unlimited possibilities and opportunities that the future might bring. The Torah instructs us not to spend our time contemplating or worrying about the future; rather, we should let life run the course set by Hashem and accept everything that happens wholeheartedly.
Additionally, Shlomo Hamelech stated, "Do not say, 'How was it that former times were better than these?' since that is not a question prompted by wisdom" (Koheles 7, 10). Chazal (Yalkut Shemoni Shmuel I on the above pasuk) tell us that Yiftach in his generation paralleled Shmuel in his generation and one must follow the judge of his generation. One shouldn't spend his time reminiscing about former glorious times. Preferring to focus on the past rather than on the present is not something which is borne out of wisdom.
Rav Wolbe elaborates on this idea too. The past is often viewed with greater clarity than the present. This is because the past has already occurred and one can clearly behold his accomplishments. In contrast, the present is vague since one does not and cannot yet know the outcome of any act he might perform. A person tends to favor dwelling on concrete accomplishments more than on unclear possibilities that might be achieved through his actions. Hence, Shlomo informs us that it is not smart to escape to the chambers of one's memory instead of concentrating on present actions.
In summation, the Torah is teaching us that one shouldn't worry about the future nor should he long for the past. Rather, one should focus all his efforts toward investing into the present. Chazal tell us that Hashem judges a person according to his present state since the present both encapsulates the past and is the seed for the future. Every day and every tefillah in Elul is an opportunity to connect to Hashem in a way that is not possible during the rest of the year. Don't worry about the future and don't live in the past - make the best out of every day as it presents itself!