So, looking at more of the materials related to Psalm 40, here are a few thoughts:
1. Some translations take "watery" as symbolic for "desolate." CSB is one of those, where they try to translate the meaning here more than the actual terms. That's one of the challenges of translation in sections of Scripture like the Psalms: poetry does not translate literally all that well. Imagery is likely used with meaning behind it, so what do you do with that?
2. Psalm 40:5 notes an important point about the works of God. There is always more that God has done than what we know. We should emphasize the works of God as noted in Scripture, as these are the ones we know in common and know without error. However, do not assume that the Scripturally recorded events are the only things God has ever done or is doing. We should be willing to listen and examine what is happening so that we can compare it to what we know of God and see if it is His work.
3. Note that for Psalm 40:10 to be true, that David did not conceal God's constant love in the great assembly, David had to confess his sins to the great assembly. We should take from this the need to be open about our sinful behavior and not conceal it, but also to put the focus where it belongs: the grace of God. David's focus shifts to the grace of God. Is your testimony about God or about you?
4. Notice also that David links his troubles with his iniquities. He is not disclaiming responsibility for his problems: he knows the source is judgment for his sins. You don't get out of responsibility just because you get into God's grace. You still have to wrestle with some of the consequences. Including finding some way to repair your relationships here: can you imagine how much David had to work through with his kids, his wives, with Bathsheba, and ultimately with the people of Israel?
5. The Psalm opens with discussing singing new songs and hymns of praise. I think we lose something we do not sing--and we lose that even if other people sing on our behalf. Singing is a good thing, and it's a long-standing habit of human beings. We should find the right things to sing about. It's also easier to sing together than recite together--you'll find that the times we recite words together, usually they are more rhythmic than normal speech. Why? Closer to singing.
And no, you don't have to hit the right notes. It helps but if we could all ease up on worrying about it, we'd be better off.