Ok, so the minor part is I I VII I and you want to go to the relative major.
One thing I should clear up before showing you how it might work is when you use the natural scale to build a chord progression, it's going to be hard to hear a clear tonal shift because the natural minor scale is just a mode of the major scale. That's where the harmonic minor scale comes in.
Back to your question. What you could do is this.
Play the Bbm Bbm Ab7 Bbm chord progression, and on the last time play Bbm Bbm Ab7. Don't go back to Bbm, instead suspend the Ab7 chord, and when you want to change to the major sound. Resolve to Dbmaj. So it would be.
| Bbm | % | Ab7 | Bbm | (repeat 3 times) | Bbm | % |Ab7 | % |Dbmaj ...etc. |
As for connecting sections. Try working on just one aspect at a time.
To change the rhythm, play though the first section you have. At the end hold out the chord, but play it with a different rhythm when you want the go to another part. Don't worry about the melody or fills. Once you got the rhythm change you want. Use the rhythm you got and work on the next section, using the rhythm.
To change chords in sections, use cadences. The above examples is this type. Ab7 to Dbmaj is a major cadence. To preform a cadence the chords move either a fourth (Ab - Eb) or fifth (Ab- Db) away. To move to a minor section, the domiannt chord of the major key (in Dbmaj, the Ab7 chord) moves up a second. Ab7 to Bbm.
For parts that are a far contrast, end the first part on a weak beat. Then start the next part on the following strong beat.
Start by writting simple songs, I know you probably want to write complex stuff, but you have to start somewhere. Plus you will probably be surprised how far a simple song will go. the more songs you complete the better you'll get, if you stay on one song to long you won't get much done.
Another thing is to listn to your favorite songs, listen to them and rewrite them. What I mean is listen to the song all the way through and put your own ideas to it. Alot of songwritters start out this way, because it's the best way to learn song structure, and it opens you up to alot.
As reassurance, Beethoven would work on a song for months before calling it finished. Your not the only one. ;)
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.