The "head first" phobia may be due to a subconscious concern about smacking face-first into the bottom of the pool, which can be a very real concern when diving from the side of a pool.
If you have a local pool you can swim laps in, you might try doing laps while snorkeling. I do this at our local YMCA, primarily to work the leg muscles with the fins, and no one seems to have any problem with using the gear in the pool. From time to time, I go ahead and submerge and skim along the bottom. For working through your phobia, you could try this and just work on a quick shallow dive and right back to the surface. This way, you're getting small doses of the phobia response, rather than a longer experience.
This is how professional therapists work with patients to get beyond phobias - small steps at a time. An arachnaphobe would be exposed to photos of spiders at first, then, when they've brought the fear response under control in that situation, to spiders safely confined behind glass. The fear is dealt with progressively until the person can handle spiders in their personal space, and eventually, until they can allow a spider to crawl on them without feeling the fear and panic of the phobia.
In most cases, the biggest step in dealing with a phobia is to understand and admit that - while the fear is real, it's an irrational fear. Knowing the reality of our fear helps us to work through it. Each time we accomplish even a minor situation where we've worked through our fear, it helps weaken that fear's hold on us. We gain confidence that we are capable of handling the thing or the situation we are afraid of, and rather than our fear mastering us, we become the master of our fears.
If you have a local pool you can swim laps in, you might try doing laps while snorkeling. I do this at our local YMCA, primarily to work the leg muscles with the fins, and no one seems to have any problem with using the gear in the pool. From time to time, I go ahead and submerge and skim along the bottom. For working through your phobia, you could try this and just work on a quick shallow dive and right back to the surface. This way, you're getting small doses of the phobia response, rather than a longer experience.
This is how professional therapists work with patients to get beyond phobias - small steps at a time. An arachnaphobe would be exposed to photos of spiders at first, then, when they've brought the fear response under control in that situation, to spiders safely confined behind glass. The fear is dealt with progressively until the person can handle spiders in their personal space, and eventually, until they can allow a spider to crawl on them without feeling the fear and panic of the phobia.
In most cases, the biggest step in dealing with a phobia is to understand and admit that - while the fear is real, it's an irrational fear. Knowing the reality of our fear helps us to work through it. Each time we accomplish even a minor situation where we've worked through our fear, it helps weaken that fear's hold on us. We gain confidence that we are capable of handling the thing or the situation we are afraid of, and rather than our fear mastering us, we become the master of our fears.
Maybe just starting in the shallow end of a pool with snorkel gear gear and getting comfortable with simply floating face-down as normal and then swimming down to touch the bottom and back up. Really just a few times and you'll be able to naturally swim down more deeply - "doing it before you know it" (btw, worked for me). You're already in good shape staying positive - good luck! 
