Hillary Clinton is not hurting Trump as much as he is hurting himself with off the cuff comments and tweets he makes daily. Another problem he has is he's not only combating Clinton but also members of his own party.
Nevertheless making statements such as "Why make nuclear weapons if we have no intention of using them?" Those kind of remarks make other nations including our allies nervous. It's also didn't help him to get into it with Muslim couple whose son gave his life to protect fellow American soldiers.
Trump needs to narrow his focus on Clinton and what (he) plans to do to jump start the economy. Unfortunately he is easily baited by everyone finds himself coming off as being unhinged or unstable. There's also the fear he knows little about foreign policy.
Having won the nomination I can't imagine he would quit in front of the whole world unless he could "spin it" in such as way to make it seem as though the whole election process is rigged...blah blah blah
The debates will be his last best shot at convincing Independents and swing states to vote for him. However I've never heard of anyone winning an election without the support their own party leaders.
The real rigged system was put in place when the RNC made all of it's candidates sign a pledge swearing they would support the nominee.
They had never done anything like this before and only did so this time in hopes of keeping Trump from running as a third party candidate. They never dreamed he would win the nomination!
So now you have all these people going back on their word after signing the pledge. (Jeb Bush, John Kasich, Marco Rubio, Ted Cruz ...etc) None of them are enthusiastically supporting the Republican nominee. The party is far from being united.
There's even talk of some elected Republicans voting for Hillary!
Clinton is just one of several obstacles that will keep him from winning. Another factor to consider is the electoral college map.
The states with the biggest populations have the most electoral votes. In the past a Republican candidate was helped by getting at least 30% of the Latino vote and winning Florida, Ohio, or PA. Trump is presently not wining any of those states and he's not likely to win among women, Latinos, Muslim Americans, or blacks. He also scores low in the polls with college educated white voters too.
That's a whole lot to overcome in 90 days!