When will I sleep better on citalopram?

Your sleeping problems should have subsided by now as most common potential side effects tend to subside within a week to 10 days when starting treatment and as you mention it has been 6 weeks you've been on this antidepressant (everyone reacts differently to medications, for some people it may take more and others less time to adjust/adapt to the medication).
If you didn't inform your Dr. about you adding Melatonin to help you sleep , I suggest you inform him/her, so if there are potential interactions your Dr. may help/prescribe you with a safer medication.

There is a moderate interaction between your antidepressant and Melatonin and 2 moderate drug and food interactions, moderate meaning:

Moderate Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.

Interactions between your drugs
Moderate
melatonin citalopram
Applies to: melatonin, citalopram

Using melatonin together with citalopram may increase side effects such as dizziness, drowsiness, confusion, and difficulty concentrating. Some people, especially the elderly, may also experience impairment in thinking, judgment, and motor coordination. You should avoid or limit the use of alcohol while being treated with these medications. Also avoid activities requiring mental alertness such as driving or operating hazardous machinery until you know how the medications affect you. Talk to your doctor if you have any questions or concerns. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.

Source and more information: https://www.drugs.com/interactions-check.php?drug_list=679-0,1548-0

As with most antidepressants there are metabolic changes and according to the literature of Drugs.com for Citalopram, common side effects that affect 1.0% to 10% of patients may have increased/decreased appetite as well as weight gain/weight loss, as I mentioned everyone reacts differently, you may or may not experience them.

Can antidepressants cause weight gain?

"While some people gain weight after starting an antidepressant, the antidepressant isn't always a direct cause. Many factors can contribute to weight gain during antidepressant therapy. For example:

Overeating or inactivity as a result of depression can cause weight gain.
Some people lose weight as part of their depression. In turn, an improved appetite associated with improved mood may result in increased weight.

Adults generally tend to gain weight as they age, regardless of the medications they take.
If you gain weight after starting an antidepressant, discuss the medication's benefits and side effects with your doctor. If the benefits outweigh the side effect of weight gain, consider managing your weight by eating healthier and getting more physical activity while enjoying an improved mood due to the medication."

Source: Mayo Clinic

Always maintain your Dr. informed of any changes you experience and how you feel with Citalopram, also if you plan to add a new medication.

If you need/decide to add an OTC medication and can not get a hold of your DR., ask your Pharmacist about potential interactions, or you may check on your own by clicking on the link provided below and following the instructions:

https://www.drugs.com/drug_interactions.html