I had a miscarriage 15 years ago. I have been wondering, will I be reunited with this little unborn soul in the next life?
First, I am sorry for the experience that you have had to go through. MIscarriage can create a unique form of sorrow that is very, very difficult to heal from.
With regard to your question: Certainly, in the case of children who die in infancy, we have specific promises through Joseph Smith and other prophets that we will eventually be reunited with them. But where miscarriages are concerned, the Church has not been as clear. I suppose that one reason for that is that we do not know for certain precisely when, during pregnancy, a spirit enters the new body. In the absence of specific revelation, the Church as a matter of policy does not record the births of stillborn children or provide proxy sealings to parents on their behalf. However, there is certainly nothing incorrect about hoping to one day be reunited with such children. No less an authority than Joseph Fielding Smith wrote:
“There is no information given by revelation in regard to the status of stillborn children. However, I will express my personal opinion that we should have hope that these little ones will receive a resurrection and then belong to us. I cannot help feeling that this will be the case.
“When a couple have a stillborn child, we give them all the comfort we can. We have good reasons to hope. Funeral services may be held for such children, if the parents so desire. Stillborn children should not be reported nor recorded as births and deaths on the records of the Church, but it is suggested that parents record in their own family records a name for each such stillborn child.” Cited in “Our Stillborn Baby”, Ensign,Feb 2006.
Speaking from personal experience, I have had quite a few close family members miscarry and share their experiences with me. One, later received very powerful spiritual assurances through the Spirit that she would indeed be reunited with that lost baby. Another, told me that she had received an equally powerful spiritual confirmation that the spirit originally intended for her pregnancy had been sent to another family, and that all was well with it. Yet a third, was certain that her fetus had never had a spirit in it at all.
Never having been pregnant myself, and not being familiar with any authoritative Church teachings in this topic; I could only suggest that you keep an open mind, seek guidance from the Spirit, and hold fast to any sacred assurances you may receive through that means.