I have had bone loss in my gums for years. My dentist says implants won't work because there isn't enough bone left.

Sorry to hear about your issues, unfortunately you need bone to place implants and it sounds like although you have been seeing a dentist every 3 months for cleaning you have waited too long and are unable to get implants.  Bone grafting does exist to help facilitate the placement of the implants.  I would seek an opinion from a periodontist or an oral surgeon to see if you are a candidate for bone grafting.  You should have some options so as not to wear removable dentures for the rest of your life

If you have lost all your teeth, unsecured, traditional dentures are a solution which in many cases has limited success. Often traditional dentures are painful, loose and unstable. However, the All on 4 | Teeth in a Day concept replaces your missing teeth with a full dental bridge supported by only four or more dental implants . With less dental implants needed, overall treatment time is reduced.The unique All on 4 treatment concept also ensures greater stability in the bone, reducing the need for bone graft surgery to increase bone volume.ADVANTAGES OF ALL-ON-4 | Teeth in a DayA cost effective solution. Only four implants are required for each jaw. With fewer implants, the cost is loweredReduced need for bone grafting. The special tilting of two of the implants ensures a secure and stable anchorage for the replaced are, typically making bone grafting unnecessary.Faster treatment and healing time. Your replacement arch can be attached to your implants immediately after insertion. There is no need to wait for healing time between surgery and tooth replacementScientifically proven and documented. The All-on-4 treatment concept is supported by good clinical outcomes from studies in which four implants have been inserted to support a full replacement arch. The tilting of implants has been used in clinical practice for over a decade with favorable results.

While it may be that you have significant bone loss due to ongoing periodontal disease it may not be a hindrance to a reconstruction with implant supported dentures.  When teeth are removed for the reconstruction, it is very likely that enough base of bone will still be available to support dental implants.  If additional bone is needed, it can be grafted to the appropriate sites.  Following implant healing, either a non-removable hybrid denture or a removable, implant supported denture may be fabricated.  These type of appliances are far more sturdy and comfortable than free floating dentures.