When to start upping stakes?

Hi, I've been trying out a new system (No, im not selling anything, it's all In-Running) for the past 3 weeks or so and its working even better than I had hoped. Seeing as initially it was just a trial run I kept stakes to a minimum averaging around £30 a bet (all place market so odds average around 1.8ish), hoping to make 4x stake a week with the idea of gradually moving up to around £200 a bet.  So far ive made around around 13x stake each week with just 1 losing day.

The problem is that I cant seem to find my bottle to up the stakes anything more than minimally (could be comfort of winning almost every day, as before this I was having quite a bad run betting prerace at higher stakes). Currently having my best week (£430 so far) yet im still placing £10 / £20 bets on odds on shots; Im hoping it'll just take more time to gain confidence (Ive never had a problem betting £200+ a race in the past) but was curious if there are any other cowards out there and how they got over it