How to fix my oil painting

I am not an expert, although I tend to scrape off the area if thick & apply a coat of Titanium white, may need 2 thin layers allowing it to dry in between. Then have another try. As you are applying thick paint any way you are obviously not worried about keeping the texture of the canvas uniform. 

From the photo I can’t actually see any issues Jen, it seems to be progressing rather well in fact! I’m not sure why you aren’t using any medium, oil paints do of course contain a certain amount of oil but I can’t paint with neat oils (I know Robert Jones does and no doubt he’ll reply also). I find that a spot or two of distilled turpentine helps with thinning them down a touch. Anyway, sanding is a bit severe but may work providing the thick paint is bone hard, as in six months old or something near. You can paint over the top but it would need to be thick paint which is perhaps counter productive. (remember the thick over thin rule with oils), generally referred to as fat over lean. Better in future to thin down your paint so this problem doesn’t occur again - I’m sure Robert and a few others such as Marjorie may have some advice to offer. Oh, and welcome to the forum, why not post some of your work on the gallery!

Difficult, but I should be inclined to paint over the top, adding stand oil or Linseed or Walnut oil if needed to make the paint more fluid, and also to fatten it up a bit.  Fat over lean is less of a necessity on rigid supports, but important on stretched canvas.  However, yes you can lightly and carefully sand those areas you wish to reduce in thickness, but Alan's absolutely right - make sure it's REALLY as dry as it is going to get at this stage, or a hideous mess could ensue, which would be a great pity in a painting as good as this.   Would your re-doing of the flowers be a major operation?  Are you proposing a very radical alteration?  If not, I'd paint over them; if you want to change them entirely, I'd (carefully - I know I'm repeating myself) sand them back.  If you're using a stretched canvas, you will need to very gentle and gradual with this.   I do use a medium - i.e. Linseed oil - but not solvents.  If I were going to use a solvent, it would be Turpentine rather than anything else - even though it's the most hazardous.  But not at this stage - a little oil at most, plus paint, either over the top of your existing flowers if the re-do isn't too radical, or on lightly sanded-down paint, would be the way I'd go.  

Yes, I had meant to mention, and as Robert rightly states, that you’d need a spot of stand oil or similar added into your mix if you go along that route. I am slightly baffled as to your reasoning behind not using any medium… speed and less detail shouldn’t be affected by thinning down your paint, in fact I’d find it a nightmare trying to push thick heavy oil paint around, although my style and subject matter is somewhat different. Robert and I differ on the use of thinners, although of course I totally agree that they are hazardous and great caution is needed, as in not breathing it in directly and having plenty of ventilation. It probably stems back to my art college training many decades ago, and I’ve never deviated from it, and as far as I know, there’s never been any issues with my oil paintings falling apart!  I must confess that health and safety back then was virtually non existent, fortunately that’s all changed! Anyway, do come back and post the completed painting, both here and on the gallery. It’s a tricky one with so many greens, my only advice would be to keep an eye on your tonal values, contrast could be the key to success here!