There are different possible causes, so it’s hard for anyone to give you solid advice. Like candi said, SIBO is one possibility. Stress obviously contributes massively to poor digestion. Have you suffered from stress in the past? Hormones out of whack can seriously mess with your health. Have you ever had any labs done?
I think anyone suspecting candida should have their thyroid function checked first. Poor function makes for a horrible uphill struggle, and people have seen symptoms like GERD and constipation disappear after treatment. You need to at least check TSH, T3 and T4 and get them all within range. There’s a trend to rely solely on TSH, so many people don’t get a proper diagnosis or even proper treatment after diagnosis.
With digestion and metabolism working at a standard rate, the rest of the process should be much easier. This is one reason to make sure you eat enough calories and carbs. Low-calorie, ketogenic diets can significantly lower T3 levels and give hypothyroid-like symptoms.