I think the best thing you can do if you are finding the process of leg-working your novel around the agents is getting stressful for you is:
1. Start writing the next novel.
2. Detach yourself emotionally and don't expect a critique.
3. Draw up a short list and a long list of agents and publishing houses you think might be suitable and then just send it out on automatic. As soon as one comes back, send the next one out. Then forget about it.
4. If you can afford it and really believe in your work, get a paid professional critique. Be careful here, however, these waters are full of sharks. However, I can personally recommend The Literary Consultancy which 100% for real and has direct links with hundreds of the main agents and publishers.
5. Don't forget that some of the world's greatest writers had their manuscripts rejected hundreds of times before they found success. Agents reject work for all kinds of reasons, as many business related as literary.
6. Keep writing.
Hope that helps and all the best.