My condolences to you and your entire family for the loss of your nephew and for the loss of your two pregnancies.
With such significant losses in such a short time, it is not surprising that the panic attacks have resurfaced along with other anxiety symptoms. You say that you keep telling yourself " I'm fine," but that undermines the legitimate need for your feelings to be acknowledged and validated. Each of these losses is significant, and each must be mourned.
I just did a google search on pregnancy loss and found numerous sites where you could get some helpful information to start the grief process and perhaps hear what other mothers in similar situations have done to help them cope.
Grief support groups are very helpful along the path to healing because everyone there can relate in some way to your situation. Hospitals often offer these groups as do many churches and synagogues, but these may or may not be for a specific type of loss. (You probably wouldn't get as much out of the group if it were specific to the loss of spouse, for example). In your case, because you have losses of different types, you may want to seek out more than one group (one for pregnancy loss and one for loss of family member or child).
Because your losses are so significant and many, it would be helpful, in addition to reading about loss and the grief process, to have someone you could talk to in person. Your primary care physician and your OB GYN, especially, should have some referral sources to guide you in the direction of a grief support group specifically for pregnancy loss. If they don't have the information, call some of the local fertility clinics; they often hold support groups for couples experiencing infertility and/or pregnancy loss.
While well-meaning others may tell you to "focus on the healthy children you have" and "don't worry, you'll be able to have another baby," these statements don't help the pain in your heart for the little ones who were lost. Your feelings about them need to be verbalized, validated, and given a special place.
As you know, there is no quick short-cut to healing, but the sooner you engage in the process, the more likely you will be able to manage your anxiety symptoms before they start to manage you and really interefere with your daily functioning.
In addition to support groups, individual counseling may be warranted. Again, it depends on the degree to which your symptoms are interfering with your ability to carry out your daily responsibilities. It sounds like you are coping as best as you can under the circumstances, but untreated grief and symptoms of depression and anxiety can become debilitating rather quickly if not treated properly. Resist the temptation to downplay your feelings and be honest about how you're really doing. You wouldn't want to have a panic attack while driving somewhere with your children, for example.
The intrusive and persistant thoughts and fears about death, accidents, and losing more family members are understandable under the circumstances, but again, if your waking hours are filled with these thoughts, it becomes agonizing, and the battle that takes place in your head to try and tame these thoughts can be entirely exhausting and time-consuming. If your discomfort in this regard is causing you additional stress, that is additional stress you don't need right now.
That leads to my final point. If you can get the panic attacks and unwanted negative thoughts under control with support groups and counseling, great. But, if not, please do not rule out medication. When prescribed and continually monitored by an appropriately licensed mental health professional, psychotropic medication can be life-saving. It does take some time and patience to get the correct medication and dosage worked out, but it can make a very big difference.
Healing happens. It takes time, and you never forget the precious ones you've lost, but you do learn to incorporate the loss into your life and move on with new hope.
You and your family will be in my thoughts, and I wish that all of you will come to a place of peace with the losses you're now experiencing.