I've played off and on for 30 years and the best I got down to was 13. I rejoined my local club when I 'retired' from work after 6 years of PD and now play an average of twice a week.
My physio has confirmed Golf is pretty much perfect for PwP. The walking, bending, rotation all keeps us active and by combining our hands in the grip we keep our co-ordination for a little longer.
Regards to Handicap, I play off 16 and am referred to as BwP (Bandit with Parkinsons). It's even been suggested I take a drugs test for the dope I take during the round (Madopar every 6 holes). Without it I am done in by the 10th.
My best round ever in 30 years was a month ago when I hit a 79 (9 over). I was feeling very shaky and weak but gave it a go. After the round I asked the pro why when I could barely grip the club and only swing at half speed was my round so good. He said that's what I should have been doing all along. To prove it, my next round I was feeling great, hit it hard and scored 101.
Maybe you're still trying to hit the ball like you used to? Since rejoining I've won a pairs comp with me scoring on 8 holes, and won two eclectic comps with best scores over several rounds.
Get out and enjoy what makes a good walk worthwhile.