I do sympathise with you, as I was in your position for years too, before finally getting sponsored at a similar age you are now.
I too was getting knocked-back at an early age, but you have to persevere.
No one cares that you went solo on yr 17th birthday, or when you got your licence. In the business of getting sponsored, it's all about what you have to offer - being 'highly motivated' and an 'excellent pilot' will just get laughed at on any application - the latter will probably see it chucked in the bin.
If you haven't got the raw materials (£££) to get your commercials, then you need bits of paper and a personality that will sell yourself well, should you get an interview.
Stop looking backwards and look ahead at how you can go from here.
I don't mean to be harsh, but these are the facts that you are faced with and you need to be harsh on yourself. The industry does not owe you a living.......
Good luck