As global hashrate goes up, then we find blocks more quickly. As we find blocks more quickly, the difficulty adjusts every 2016 blocks in order to maintain the average block time at 10 minutes. It does this by reducing the target number miners are trying to find. By making the target smaller, we need more hashes in order to find a number which meets that target, hence it is more difficult.

Sometimes the hashrate falls and so the difficulty adjusts downwards (making the target larger), but it's been pretty close to an exponential trend upward for many years. You can see all difficulty adjustments here: https://btc.com/stats/diff

This, however, is all to do with mining blocks, which uses hash functions. This is completely separate to the security of private keys, which uses elliptic curve mathematics. The security of your private keys will be 128 bits regardless of where the network as a whole has 100 hashes per second or 100 trillion hashes per second.