Dublinpilot made a very good point about GPS and making mistakes by turning early/late. Keeping a record of your actual track will point out where exactly your problem lies. I had the exact same problem, I used a sportstracker app on my phone - not only it showed my route, it also recorded my ground speed which was handy assesing the effects of wind etc, I only needed one look at it and that fixed my problems for good
I have mixed feelings in the use of aids ! I know when I learnt to fly we were forbidden to use any navaids whatsoever
There is some truth to the statement that pilot aids should be a compliment to your flying skills and not a crutch for a lack of those skills
You cannot always rely on pilot aids you can on basic skills and it is those basic skills which need to be solid!
At some point a PPL is issued which licences you to fly people who are totally relying on your skills for their lives
If you need pilot aids to cover a lack of basic skills I petty the PAX