Hi Cathie,
frustrations, anger and lack of progress go hand in hand with making music *eeek*
Sometimes when I have the melody (tune) I go to another software in my DAW - the tool is Scaler 2, where you can enter the midi notes from the piano/synth, and it'll determine which key fits the best to your melody. This is sometimes very helpful.
When it comes to recording in parts - I suggest you go to your DAW (Ableton, Logic Pro X or likewise). In Logic I can create a take folder and record the same piece over and over again, and end up with maybe 15-20 vocal takes of the same part of the song. Afterwards I can comb (select) from each take and assemble the vocal from there. (I'm not sure which other DAW offer this feature). When recording vocals, I never play at the same time - I need to be 100% focused, when I sing. I have no stage or live experience....so that is why, I do not master singing and playing at the same time.

BIAB has so many capabilities, and my main frustration is, that it doesn't always fit to my way of thinking.