You're at a point where you need to sorta train yourself.. Sure, a coach can give training advice, and some technique, but you already have the core components of double, tripple, etc... Twisting.
What you don't have is confidence in youself that you can do it. I think that your 1 and 1/2 is your key. Are you just landing that? Or do you have a smidge of air time? Can you open high and land light?
If you can, do so! Then, do a hop 1/2 jump. Little by little, you will see and feel where that extra 1/2 turn can come. A trampoline will help tons, as will a tumble track. A coach could help with an overhead rig if its available when you want to turn it fully.
You will get it if you want it. Be patient and do it right, and you will be back here sooner than you think asking if tripple fulls are worth competing (maybe ).
Ryan is right, if you can't do it, you either have a mental block or you simply do not have enough time in the air to complete the extra half. Try starting on a trampoline track to give you the extra height an time to wrap in two twists. Make sure you have a good clean consistent 1 1/2. Practise is the key, and with your coach's help you will get it eventually. Best wishes x