Since you have been examined by a doctor and there are no problems, you can expect to be successful in getting a pregnancy through natural means.
The first thing is that you are still breastfeeding. Breastfeeding decreases fertility, this is known. You have to stop breastfeeding to increase your fertility. This alone will help greatly.
You are already aware of your luteal phase, you should follow a targetted method of sex during your fertile period to increase the chances of pregnancy.
There are several supplements which are known to help fertility, though whether the effect is on the luteal phase or not is not known.
The most important is Acetyl L-Carnitine. the second is Co-enzyme Q. Besides these, normal vitamin supplements like Vit. B, C and D are important.
Try these out, stop breastfeeding - you should get pregnant soon.
Wish you best of luck.
Hope this helps.
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