This letter is to formally announce my resignation from [company name], starting today. My last day will be [two weeks from today].
I have not accepted a job with another company as of yet. My decision is based on the uncertainty of the future of my career at [company name]. I have tried to discuss my career advancement in the past, with no real results.
I would welcome the opportunity to discuss with you a possible career future, assuming you are interested in keeping me employed with your company.