What's gone wrong?

[Win]

I thought I had finally found a winning strategy for UK horse races. From April to July I was winning every week and only had 19 losing days out of 116. I was only winning on 55% of races, but my wins were always bigger than my losses.
Since 1st August though, I've only had 5 winning days. Losing days are losing more than winning days win. And I'm only winning on 46% of races.
Does that mean that my strategy was just luck?
Or am I just going through a losing period. In 18 days I've lost 12% of what I won in April to July.
Or does UK horse racing change in different months and I should put my strategy aside and try it again next year?
After years of having more downs than ups, I thought I had finally cracked it. But now I'm wondering.
My strategy was 100% automated because I have never been able to trust myself trading manually.
All thoughts welcome please.