For the first time, when I move to cryptocurrency world (March 2016) the btc price were around $400... it was risen up to $17000 few days ago... 17000/400 x 100 = it's 4520% growth in 21 months.
There's a popular statement such as, the thing what it is existing next to the risk is the success.
Should I believe it and do what I didn't do 21 months ago?
I think you missed the bus again, Bitcoin went down again and it has rebounded very fast, I don't know what you did 21 months ago, but do follow this formula, buy at every dip and sell when the price's shoot through the moon, if you need the money or else just hodl it. If you buy before 28th you may get free coins (segwit2x), though I'm not sure it's happening. Or go to the collectibles and buy a coin that had btc, you'll get all fork coins to that should make up for your 21 month's stint.