This situation is probably high when Bitcoin is showing signs of about to increase in price. the price level in 2020 can be higher than in 2017 so let's wait and see what happens.
Yes, besides waiting, it's good to buy as much as you can if you really believe in the Bitcoin price hike that will surpass 2017 in 2020, actually this looks like an opportunity, but not many are taking advantage of it.
Already the price of Bitcoin is very close to previous ATH, now people are afraid of buying at the current value because previously they faced lot loss due to people bought the Bitcoin at a higher rate. Instead of investing all our amount at a higher price, it is good to invest less amount. Otherwise, we might lose our investment completely.