My website's SERP is drop after I do it.

If your site was already ranking well with the previous link profile, and when you add new links, chances are, those links may not be doing good for your site, especially if they are from over-spammed sites. Big G appears to be keeping track on over-spammed sites, and having a link from these sites would definitely drag your site down. So if this is the case, you can remove the forum profile and try to remove the blog comment links.
Otherwise, if the forum is a powerful one and well regulated (yes Big G knows this), and has good user engagement, then that forum profile link shouldn't be an issue. The same goes with the blog comment - if the comment is located on a well-regulated site, with very little spam and high user engagement, then this shouldn't be an issue as well too.
Just go back to the links that you have built recently, and try examining those in depth.

SERP is the search engine result page. When user enters the search query that time search engine results the web page as output. It results the unique pages though you have entered the same keywords but different phrases. The result used to vary because of the various daily to daily researches applied by the search engine.

It has two types of results as organic result and paid result. Organic result is the result which returned by the search engine as result of the search engine algorithm. Paid result are displayed because the owner of the web-pages has paid the certain amount to display their web-pages as the result.

Here are tips to generate the sitelinks -

1. Select the proper site name four website or brand name for your blog.
2. Make sure you have the quality contents which will attract the users and make sure to update them periodically to get the traffic.
3. Create the information page that is About us, Contact us, Location pages for your website.
4. Make sure your website has proper structure so user feels easy to navigate between the pages with ease.
5. Add the internal links properly in between the web pages of your website.
6. Add sitemap for your website.