Imagine a guy sitting in yeshiva who has a mussar seder just before ma’ariv. During that seder he engages in deep introspection and comes face to face with all his flaws, with all his shortcomings; he meditates on where he is now vs. where he should be. Then he davens ma’ariv – “Refainu… Bareich aleinu…” etc. How can the same person who spent 20 minutes thinking about how far from his ideals then turn around and start making requests from Hashem?
The question is not a question. Tefilah has no prerequisites. You can ask Hashem for anything and everything no matter where you are holding – in fact, that’s exactly what Hashem wants. Does a child think about what he/she deserves before asking a parent for it? Of course not. So too, tefilah allows a person to approach Hashem warts and all, whether deserving or not, and ask for the world.
Mahral explains that “katonti m’kol hachassadim” is an assessment of reality.
“Hatzileinu na m’yad achi m’yad eisav… v’atah amarta heitev eitiv imach” is a tefilah.
I would like to suggest another answer to the Ramban’s question. Many of the meforshim struggle to explain the double-language of “heiteiv eitiv.” The Tiferes Shlomo writes (he connects it with yichudim - I'm simplifying the point) that Hashem promised not only that he would do good for Ya'akov, i.e. "heiteiv," but more than that, he promised "eitiv imach," I will do good with you, i.e. through you = “al yadcha” (in the Tif Shlomo’s words). In other words, Ya'akov would be a vehicle to bring tovah to others.
When a person is by himself, even when Hashem made a promise of tovah to that individual, shema yigrom hacheit needs to be taken into consideration.
However, when a person is a connected with others and he/she acts as a conduit of tovah to others, i.e. there is an "eitiv imach," through the individual, then apparently shema yigrom hacheit doesn't matter because any tovah Hashem gives is not a gift just to the individual, dependent on his/her personal merits, but is something that is by deifinition going to be shared and spread to all those the individual supports and encounters.
Ya'akov when speaking about himself could say, "Katontim m'kol hachassadim," but at the same time, as a husband and father, as a conduit of tovah to others, he could call on the promise of "heiteiv eitiv eimach" without worrying about being turned back by Hashem.