There are three parts to tefila 1) praise of Hashem, because without our recognition of Him as the Source of all good, there is no point to any further tefila 2) bakasha, our personal requests and heartfelt pleas, which are considered to be the essence of tefila and 3) hodaa, expressions of our appreciation to Hashem for all He has done, does, and will do for us on a daily basis.
Theoretically, if one has engaged in all three of these aspects of tefila, he has fulfilled the essential mitzvah of being mispallel. However, since the Anshei Knesset Hagedola instituted formal tefila, in a pattern that we still use today, it is good to follow that specific form, as they truly included every aspect of life in it. The Shemonah Esrai truly addresses all our needs, including leaving an area for private and personal bakasha to Hashem. What we sometimes tend to forget, is we do not have the right to feel a sense of entitlement where Hashem is concerned.Sometimes He doesn’t give us what we think we want and need, even when it is very, very important to us (like a refuah shelaima). It’s very hard to accept and understand that, but that is the middah that I am working on very strongly. Bitachon is easy when everything is OK.