Cast Me Not

Psalm 51:10-12 (NASB)
10) Create in me a clean heart, O God,
And renew a steadfast spirit within me.
11) Do not cast me away from Your presence
And do not take Your Holy Spirit from me.
12) Restore to me the joy of Your salvation
And sustain me with a willing spirit.

Pslam 51 is reguarded as a prayer of David after Nathan had came to him and let him know God knew his sin. Verses 10-12 is a cry to purity for us as His children today, as we seek Him for a pure heart, a spirit of faithfulness and a willing spirit of service.
In verse 10 the word "Create" is important as it is asking for something new, which cannot emerge from what now is, and which only God can fashion.
As in the very beginning in Genesis 1:1-2 when God created the heavens and earth from something that was not there, (the earth was formeless and empty, darkeness was over the surface of the deep).
When God purifies his people He must create something new from our dark, formless, empty and sinful hearts.
Verse 11 the two request are essentially one in the same. David recalls the rejection of Saul and pleads for God not to take away his spirit, by which He had equipped and qualified him for royal service. By his Spirit, God had performed his purposes in creation and redemption, equipped His servants for their appointed tasks, inspired his prophets and directed their ministries.
Verse 12 The restoration of the joy of salvation and a willing spirit of service can only happen if God does not reject or take His Spirit from us. If granted the joy of His salvation will return to gladden our troubled souls which is our sustainance.