I have a great customer that got me to do a cake for her about a year ago. I had suffered a HUGE burnout prior to that and quit doing cakes all together for about a year. But she begged me to to do one for her daugheter's engagement party so I did. Since I wasn't planning on really getting back into the business, I kinda did it as a favor and gave her a really discounted price (yes.....spare the lectures....BIG MISTAKE.. I know). So this cake really rekindled my love for cake decorating and I deciced to get back into it. This customer has sent me alot of business and continues to order cakes from me...bc I discounted her so much with the first cake I've kinda crept up the cost slowly for all subsequent orders. She has never questioned it, and doesn't seem to mind at all. I just finished the groom's cake for her daughter's wedding and I did charge her full price... Again, she never questioned, and didn't flinch when she paid me.... But I can't help feel super guilty bc I know I prob should have addressed this with her. I know this is really unprofessional and it is not usually how I operate. But, when it comes to people I know, I get sooo sheepish about the money situation. I thought about writing her a thank you note after the wedding for all the business she's sent me and for rekindling my love for the art and maybe addressing the price increases with her.........or should I just leave it alone?? 
A thank you note for all the business would be totally appropriate. Don't try to "explain" the cost increases, the prices of everything has gone up and this can be your "reason", not that you need one. Don't give her anything free or discounted. Maybe a small 6 inch cake in her fave flavor on her birthday would be a nice touch, tho.
Do you get a postcard with an apology from the grocery store when their prices go up? Or the gas station?
The way I look at it - if your customer is satisfied with your price, there's no need for you to explain it to her. If she'd brought it up first, sure, tell her that you'd given her a discount previously, but since it doesn't sound like she has an issue with your current pricing then I wouldn't open that line of dialog.
It sounds like you want to thank her on a more personal level for the rekindling. I don't know that I'd tell her how burnt out you were; that could *potentially* open up questions in her mind as to whether you'll burn out again and may make her question future orders. Instead, if you want to express some appreciation, perhaps a small "thank you" card just for her loyal business and referrals, and perhaps include a gift certificate for a free 8" minimally decorated cake...a 'dinner cake' or some such?
Appreciate your customers, thank them for their service, but do it professionally. I'd be a little weirded out if my grocery store manager thanked me for rekindling his love of butter and eggs because I've bought so much of them, y'know? 
Wait...she's NOT complaining about the gradual price increases you have been doing for her and you have a problem
You really need to let this go. Obviously if she had a problem with it she would have either already brought it up in conversation with you or taken her business somewhere else. Perhaps she just figures that since the cost of everything else has gone up (her groceries, her gas, her haircuts, etc.) that it's only natural for there to be a price increase with your business.
Seriously, stop feeling guilty.
I don't think I would bring it up, there doesn't seem to be an issue from what you are telling us. I mean Wendy's doesn't consult with me when they raise the prices on their hamburgers, its just business.
If you want to send her a thank you for all the business then I would say go for it, but don't bring up prices, that would only make it more ackward. If you really feel horrible, you shouldn't, but if you do just stick in a certificate for a discounted cake on her next order.
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