Famous phrase for something which is forbidden or impossible, but can be done if desired enough

Solution 1:

You may use: where there's a will there's a way:

• used to mean that if you are determined enough, you can find a way to achieve what you want, even if it is very difficult

(Cambridge Dictionary)

Also:

will power:

• The strength of will to carry out one's decisions, wishes, or plans.

• You may say that it is only a question of will power to achieve to do something.

(AHD)

Solution 2:

If "very desired" can be approximated to "having faith", you may use "Faith will move mountains". This also goes quite well with the alternative already considered ("The only way to achieve the impossible, is to believe it's possible.").

TFD(idioms):

Faith will move mountains.
Prov. If you believe in what you are doing, you can overcome any obstacle. (Sometimes refers to faith in God.)
Jane's faith in her cause could move mountains.
You may feel disheartened sometimes, but remember that faith will move mountains.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

Solution 3:

To use it as an epigraph, you’d probably first have to decide whether to use the Latin or English version (although using both could be cool) and then whether to attribute it to Hannibal, ... Philip Sidney, ... or no one at all; but I think the notion of “not knowing the meaning of impossible” would be relevant and captured well by:

Aut viam inveniam aut faciam:
I'll either find a way or make one.

(links to softschools.com; Wiikipedia; motivated.us; brainyquote.com; and boardofwisdom.com, respectively)

Solution 4:

Consider the phrase doing the impossible.

Impossible adjective 1 Not able to occur, exist, or be done. ‘An impossible dream, I told myself, but it was what I wanted.’ - ODO

Here's a similar quote:

Difficult is done at once, the impossible takes a little longer, the proverbial saying, late 19th century; in the first recorded usage, Trollope's novel Phineas Redux (1873), the words are attributed to ‘a French Minister’, the French statesman Charles Alexandre de Calonne (1734–1842), who is said to have responded, ‘Madame, si c'est possible, c'est fait; impossible? cela se fera [Madam, if a thing is possible, consider it done; the impossible? that will be done].’ In modern times, the US Armed Forces have taken as their slogan, ‘The difficult we do immediately, the impossible takes a little longer.’ - encyclopedia.com

And here's an example used in the context of technology:

The startup that's doing the impossible - TechRepublic

Of course, if something is truly impossible, it literally cannot be done. These sayings can be taken as a form of bravado, or they can be using impossible in the sense of seemingly impossible or perhaps hitherto impossible.

Solution 5:

In addition to Josh's answer, depending on what it is that fuels the desire for something difficult or forbidden, to be nonetheless done (specifically, being in dire straits or having no other resort) you could use needs must (or in full, needs must when the devil drives).

Meaning

Necessity compels. In current usage this phrase is usually used to express something that is done unwillingly but with an acceptance that it can't be avoided; for example, I really don't want to cook tonight, but needs must, I suppose.

Origin

The phrase is old. In earlier texts it is almost always given in its fuller form - needs must when the devil drives. That is, if the devil is driving you, you have no choice. This dates back to Middle English texts, for example Assembly of Gods, circa 1500:

"He must nedys go that the deuell dryues."

Shakespeare used the phrase several times; for example, in All's Well That Ends Well, 1601:

Countess: Tell me thy reason why thou wilt marry.
Clown: My poor body, madam, requires it:
       I am driven on by the flesh; and he
       must needs go that the devil drives.

As quotations go, you have "They didn't know it was impossible, so they did it" by Mark Twain, or "Nothing is impossible to a willing heart" by John Heywood.

A more religious take is that nothing is impossible to God, while a cynical person will observe that very little is impossible to those who don't have to do it themselves.

A somewhat different (and more ambiguous) view and quotation is supplied by Guillaume Apollinaire, Come to the edge, and is usually shortened in "he pushed them, and they flew". One way of looking at it is that if one desires it enough, he can fly, even if he was afraid and thought it impossible; so, it might be a good match for your question.