We have an ex-racer TB and when we got her, she hadn't been ridden for 6 years (and prior to that only on the racetrack) so we started her off lunging her with the saddle for a long time - then had the stirrups flapping next to her whilst lunging - then my son laid across her back whilst she was stood still then my son sat on her - which she didn't mind at all (but this was my youngest son). BUT if my eldest son gets on her and uses his legs, it's a different story - she'll go backwards, sidewards and has bucked him off. We were told (by a recehorce rescue worker) that when they are trained as racehorses, jockeys don't use their legs (watch a race, and see where their legs are) so she just isn't used to any leg use at all.
She's just foaled so we'll be sending her away locally to be re-backed next year (going to give her 12 months with the foal before we work her) so i'd just go with the professional angle personally but I have no experience with training horses at all so I've no other option
Good luck - I think every horse deserves a chance - but make sure your safety comes first xx