As Mark explained, you may want to double-check whether your earlier attempt to give up your citizenship was effective. In particular, applying for a “libération des liens d'allégeance envers la France” requires submitting the usual documents to establish your identity (birth certificate…) and proof of another citizenship. Surrendering your passport wouldn't be enough so I suspect the consulate may have simply ignored it at the time.
What you wrote in your letter is not that important, the French state would in any case have to take a decision confirming it. That decision would be published in the Journal Officiel and sent to you. In other words, if you never received anything and cannot find it in the JO, you're still French. A major caveat is that I don't know how to search for individual decisions by name, it seems you need to know the date of a decision to look it up. These decisions are pretty rare (around 100 per year), maybe your consulate can help you find a list? It should be reasonably easy to scan all those of the period 2008-2012 or so and check if your name is in there.
Another approach would be to secure all the necessary documents (if you and your father are both born in France, both of your birth certificates would be enough) and apply for a passport or for a Certificate de nationalité française, maybe with a letter explaining that you are unsure whether you relinquished your citizenship in 2008. If the consulate is satisfied that you haven't and issues the document then everything else is moot.
Is regaining French citizenship complex or easy?
There is a procedure to regain French citizenship but it is not terribly easy and only open to people who reside in France. One requirement is that France should be the “main center of your interests”. What that means in practice is for example that having a spouse who lives abroad while you are in France trying to regain your citizenship might lead to a refusal.
Other than that, all the usual procedures to gain French citizenship would still be open to you, at least in theory. Most of them require living in France for a number of years; having a job and speaking French is almost always required. Beyond these (arguably demanding) requirements, it's not especially complicated as far as citizenship procedures go, certainly easier than in many other European countries. But if you live in France and qualify for naturalisation, it would make sense to apply for “reintegration” instead.
The main exception is if you are married to a French citizen. Many of the requirements, including residence, are lifted in this case and it may be possible to become French without leaving the US. Speaking some French would still be a requirement.
