Matching up to the Index - STI ETF

With the bull run of the stock market last year, many have shown their positive results of their portfolio and I started investing at around May 2017. However, my portfolio in 2017 did not match up to the STI ETF as seen below.

With the current down spiraling price of Design Studio, my portfolio has taken a hit.
What I hope to do is to increase my holdings of the STI ETF since I haven't been able to beat it. Currently, my portfolio has about 200 ES3 (SPDR STI ETF) in my CDP and 316 G3B (NikkoAM STI ETF) from POSB RSP. I am planning to increase ES3 to a larger proportion of my portfolio.